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14 Apr 2010

Volume 132, Issue 14, Articles (14xxxx)

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J. Chem. Phys. 132, 144102 (2010); http://dx.doi.org/10.1063/1.3352565 (13 pages)

William J. Glover, Ross E. Larsen, and Benjamin J. Schwartz
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Communications: Evidence for the role of fluctuations in the thermodynamics of nanoscale drops and the implications in computations of the surface tension

José G. Sampayo, Alexandr Malijevský, Erich A. Müller, Enrique de Miguel, and George Jackson

J. Chem. Phys. 132, 141101 (2010); http://dx.doi.org/10.1063/1.3376612 (4 pages) | Cited 5 times

Online Publication Date: 9 April 2010

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Test-area deformations are used to analyze vapor-liquid interfaces of Lennard-Jones particles by molecular dynamics simulation. For planar vapor-liquid interfaces the change in free energy is captured by the average of the corresponding change in energy, the leading-order contribution. This is consistent with the commonly used mechanical (pressure-tensor) route for the surface tension. By contrast for liquid drops, one finds a large second-order contribution associated with fluctuations in energy. Both the first- and second-order terms make comparable contributions, invalidating the mechanical relation for the surface tension of small drops. The latter is seen to increase above the planar value for drop radii of ∼ 8 particle diameters, followed by an apparent weak maximum and slow decay to the planar limit, consistent with a small negative Tolman length.
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68.03.Cd Surface tension and related phenomena
61.20.Ja Computer simulation of liquid structure
51.30.+i Thermodynamic properties, equations of state
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Communications: Photoinitiated bond dissociation of bromoiodomethane in solution: Comparison of one-photon and two-photon excitations and the formation of iso-CH2Br–I and iso-CH2I–Br

Kuo-Chun Tang, Jian Peng, Kenneth G. Spears, and Roseanne J. Sension

J. Chem. Phys. 132, 141102 (2010); http://dx.doi.org/10.1063/1.3374680 (4 pages)

Online Publication Date: 14 April 2010

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Broadband UV-visible femtosecond transient absorption spectroscopy was used to monitor the excited state photochemistry of CH2BrI following one-photon excitation at 266 or 271 nm and two-photon excitation at 395 or 405 nm in 2-butanol. The results for one-photon excitation agree with earlier studies in acetonitrile, showing clear formation of iso-CH2Br–I following cleavage of the C–I bond. In contrast, two-photon excitation at 395 nm results in the appearance of a blueshifted photoproduct absorption band assigned to formation of iso-CH2I–Br following cleavage of the C–Br bond. The results are discussed in the context of prior experimental and theoretical work and the prospects for optical control of bond cleavage.
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82.50.Pt Multiphoton processes
82.50.Hp Processes caused by visible and UV light
33.20.Lg Ultraviolet spectra
33.20.Kf Visible spectra
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.70.Jg Line and band widths, shapes, and shifts
33.15.-e Properties of molecules
33.80.Gj Diffuse spectra; predissociation, photodissociation
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First principles multielectron mixed quantum/classical simulations in the condensed phase. I. An efficient Fourier-grid method for solving the many-electron problem

William J. Glover, Ross E. Larsen, and Benjamin J. Schwartz

J. Chem. Phys. 132, 144101 (2010); http://dx.doi.org/10.1063/1.3352564 (11 pages) | Cited 1 time

Online Publication Date: 8 April 2010

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We introduce an efficient multielectron first-principles based electronic structure method, the two-electron Fourier-grid (2EFG) approach, that is particularly suited for use in mixed quantum/classical simulations of condensed-phase systems. The 2EFG method directly solves for the six-dimensional wave function of a two-electron Hamiltonian in a Fourier-grid representation such that the effects of electron correlation and exchange are treated exactly for both the ground and excited states. Due to the simplicity of a Fourier-grid representation, the 2EFG is readily parallelizable and we discuss its computational implementation in a distributed-memory parallel environment. We show our method is highly efficient, being able to find two-electron wave functions in ∼ 20 s on a modern desktop computer for a calculation this is equivalent to full configuration interaction (FCI) in a basis of 17 million Slater determinants. We benchmark the accuracy of the 2EFG by applying it to two electronic structure test problems: the harmonium atom and the sodium dimer. We find that even with a modest grid basis size, our method converges to the analytically exact solutions of harmonium in both the weakly and strongly correlated electron regimes. Our method also reproduces the low-lying potential energy curves of the sodium dimer to a similar level of accuracy as a valence CI calculation, thus demonstrating its applicability to molecular systems. In the following paper [ W. J. Glover, R. E. Larsen, and B. J. Schwartz, J. Chem. Phys. 132, 144102 (2010) ], we use the 2EFG method to explore the nature of the electronic states that comprise the charge-transfer-to-solvent absorption band of sodium anions in liquid tetrahydrofuran.
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71.15.-m Methods of electronic structure calculations
68.43.-h Chemisorption/physisorption: adsorbates on surfaces
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
34.70.+e Charge transfer
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.20.Kh Potential energy surfaces for chemical reactions
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
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First principles multielectron mixed quantum/classical simulations in the condensed phase. II. The charge-transfer-to-solvent states of sodium anions in liquid tetrahydrofuran

William J. Glover, Ross E. Larsen, and Benjamin J. Schwartz

J. Chem. Phys. 132, 144102 (2010); http://dx.doi.org/10.1063/1.3352565 (13 pages) | Cited 3 times

Online Publication Date: 8 April 2010

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Gas-phase atomic anions lack bound electronic excited states, yet in solution many of these anions exhibit intense absorption bands due to the presence of excited states, referred to as charge-transfer-to-solvent (CTTS) states that are bound only by the presence of the solvent. CTTS spectra thus serve as delicate probes of solute-solvent interactions, but the fact that they are created by the interactions of a solute with many solvent molecules makes them a challenge to describe theoretically. In this paper, we use mixed quantum/classical molecular dynamics with the two-electron Fourier-grid (2EFG) electronic structure method presented in the previous paper [ W. J. Glover, R. E. Larsen, and B. J. Schwartz, J. Chem. Phys. 132, 144101 (2010) ] to simulate the CTTS states of a sodium anion in liquid tetrahydrofuran, Na−/THF. Since our 2EFG method is based on configuration interaction with single and double excitations in a grid basis, it allows for an exact treatment of the two valence electrons of the sodium anion. To simulate Na−/THF, we first develop a new electron–THF pseudopotential, and we verify the accuracy of this potential by reproducing the experimental absorption spectrum of an excess electron in liquid THF with near quantitative accuracy. We also are able to reproduce the CTTS spectrum of Na−/THF and find that the CTTS states of Na− exhibit a Rydberg-like progression due to the pre-existing long-range solvent polarization around the anion. We also find that the CTTS states are highly mixed with the disjoint electronic states supported by naturally occurring solvent cavities that exist in liquid THF. This mixing explains why the solvated electrons that are ejected following CTTS excitation appear with their equilibrium absorption spectrum. The mixing of the CTTS and solvent-cavity states also explains why the recombination of the electron and its geminate Na0 partner occurs on slower time scales when photoexciting in the blue compared to in the red of the CTTS band: blue excitation accesses CTTS states whose charge densities lies further from the position of the anion, whereas red excitation accesses CTTS states that lie primarily within the anion’s first solvation shell. Finally, we see that the radial character of the CTTS states near the Na+ core matches that of Na0, explaining why the spectrum of this species appears instantly after photoexciting Na−.
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71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations
71.10.Li Excited states and pairing interactions in model systems
71.15.-m Methods of electronic structure calculations

Steady shear rheometry of dissipative particle dynamics models of polymer fluids in reverse Poiseuille flow

Dmitry A. Fedosov, George Em Karniadakis, and Bruce Caswell

J. Chem. Phys. 132, 144103 (2010); http://dx.doi.org/10.1063/1.3366658 (14 pages) | Cited 4 times

Online Publication Date: 8 April 2010

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Polymer fluids are modeled with dissipative particle dynamics (DPD) as undiluted bead-spring chains and their solutions. The models are assessed by investigating their steady shear-rate properties. Non-Newtonian viscosity and normal stress coefficients, for shear rates from the lower to the upper Newtonian regimes, are calculated from both plane Couette and plane Poiseuille flows. The latter is realized as reverse Poiseuille flow (RPF) generated from two Poiseuille flows driven by uniform body forces in opposite directions along two-halves of a computational domain. Periodic boundary conditions ensure the RPF wall velocity to be zero without density fluctuations. In overlapping shear-rate regimes the RPF properties are confirmed to be in good agreement with those calculated from plane Couette flow with Lees–Edwards periodic boundary conditions (LECs), the standard virtual rheometer for steady shear-rate properties. The concentration and the temperature dependence of the properties of the model fluids are shown to satisfy the principles of concentration and temperature superposition commonly employed in the empirical correlation of real polymer-fluid properties. The thermodynamic validity of the equation of state is found to be a crucial factor for the achievement of time-temperature superposition. With these models, RPF is demonstrated to be an accurate and convenient virtual rheometer for the acquisition of steady shear-rate rheological properties. It complements, confirms, and extends the results obtained with the standard LEC configuration, and it can be used with the output from other particle-based methods, including molecular dynamics, Brownian dynamics, smooth particle hydrodynamics, and the lattice Boltzmann method.
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47.57.Qk Rheological aspects
47.57.Ng Polymers and polymer solutions
47.50.Cd Modeling
47.15.Rq Laminar flows in cavities, channels, ducts, and conduits
47.60.Dx Flows in ducts and channels

Basis set consistent revision of the S22 test set of noncovalent interaction energies

Tait Takatani, Edward G. Hohenstein, Massimo Malagoli, Michael S. Marshall, and C. David Sherrill

J. Chem. Phys. 132, 144104 (2010); http://dx.doi.org/10.1063/1.3378024 (5 pages) | Cited 62 times

Online Publication Date: 8 April 2010

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The S22 test set of interaction energies for small model complexes [ Phys. Chem. Chem. Phys. 8, 1985 (2006) ] has been very valuable for benchmarking new and existing methods for noncovalent interactions. However, the basis sets utilized to compute the CCSD(T) interaction energies for some of the dimers are insufficient to obtain converged results. Here we consistently extrapolate all CCSD(T)/complete basis set (CBS) interaction energies using larger basis sets for the CCSD(T) component of the computation. The revised values, which we designate S22A, represent the most accurate results to date for this set of dimers. The new values appear to be within a few hundredths of 1 kcal mol−1 of the true CCSD(T)/CBS limit at the given geometries, but the former S22 values are off by as much as 0.6 kcal mol−1 compared to the revised values. Because some of the most promising methods for noncovalent interactions are already achieving this level of agreement (or better) compared to the S22 data, more accurate benchmark values would clearly be helpful. The MP2, SCS-MP2, SCS-CCSD, SCS(MI)-MP2, and B2PLYP-D methods have been tested against the more accurate benchmark set. The B2PLYP-D method outperforms all other methods tested here, with a mean average deviation of only 0.12 kcal mol−1. However, the consistent, slight underestimation of the interaction energies computed by the SCS-CCSD method (an overall mean absolute deviation and mean deviation of 0.24 and −0.23 kcal mol−1, respectively) suggests that the SCS-CCSD method has the potential to become even more accurate with a reoptimization of its parameters for noncovalent interactions.
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31.15.bw Coupled-cluster theory
31.15.E- Density-functional theory
02.50.-r Probability theory, stochastic processes, and statistics
31.15.xp Perturbation theory

Hilbert-space treatment of incoherent, time-resolved spectroscopy. I. Formalism, a tensorial classification of high-order orientational gratings and generalized MUPPETS “echoes”

Mark A. Berg

J. Chem. Phys. 132, 144105 (2010); http://dx.doi.org/10.1063/1.3327760 (14 pages) | Cited 4 times

Online Publication Date: 8 April 2010

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Time-resolved spectroscopy that uses more than one incoherent excitation, and thus has multiple periods of time evolution, is becoming more common. A recent example is multiple population-period transient spectroscopy (MUPPETS), which is implemented as a high-order transient grating. In this paper, a formalism is developed to treat these types of incoherent spectroscopy in a manner that parallels the Liouville-pathway formalism used to treat multidimensional coherent spectroscopy. A Hilbert space of incoherent (population) states is defined and general expressions for transition and time-evolution operators acting on these states are derived from the corresponding quantum operators. This formalism describes incoherent experiments that involve an arbitrary number of temporal dimensions in terms of pathways through the Hilbert space. Each pathway is associated with a multiple-time rate-correlation function. Previous work has shown that these multiple-time correlation functions can measure heterogeneity in electronic-relaxation rates. Thus, they are an analog of coherent “echo” experiments, which measure heterogeneity in frequencies. We show that similar “MUPPETS echo” experiments can be done on any incoherent variable. For a dilute molecular solute, the Hilbert-space method leads to a systematic treatment of multidimensional transient gratings. The extension of irreducible-tensor methods to the incoherent Hilbert space results in a classification of orientational gratings of arbitrary order. The general methods developed in this paper are applied more specifically to single-photon, dipole transitions in the following paper.
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82.53.Kp Coherent spectroscopy of atoms and molecules
78.47.D- Time resolved spectroscopy (>1 psec)
82.20.Pm Rate constants, reaction cross sections, and activation energies

Hilbert-space treatment of incoherent, time-resolved spectroscopy. II. Pathway description of optical multiple population-period transient spectroscopy

Mark A. Berg

J. Chem. Phys. 132, 144106 (2010); http://dx.doi.org/10.1063/1.3327781 (15 pages) | Cited 4 times

Online Publication Date: 8 April 2010

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This paper applies the general methods developed in the previous paper (Paper I) to the case of one-photon, dipole transitions of a molecular solute. The results generalize transient-grating spectroscopy to an arbitrarily number of dimensions. Transition and detection operators are derived, and their matrix elements are calculated in the complex-valued basis set developed in Paper I. Selection rules make it possible to analyze which incoherent pathways, and thus which correlation functions, contribute to an N-dimensional multiple population-period transient spectroscopy experiment. Irreducible-tensor methods allow the amplitudes of the contributing orientational-correlation functions to be calculated for arbitrary polarization conditions. A second-rank polarization tensor is used to describe the polarization of the pair of beams creating or detecting a grating. Several known results for one-dimensional experiments are rederived in this formalism to provide examples of its use.
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82.53.Kp Coherent spectroscopy of atoms and molecules
07.60.Rd Visible and ultraviolet spectrometers

Finite-size scaling study of the vapor-liquid critical properties of confined fluids: Crossover from three dimensions to two dimensions

Yang Liu, Athanassios Z. Panagiotopoulos, and Pablo G. Debenedetti

J. Chem. Phys. 132, 144107 (2010); http://dx.doi.org/10.1063/1.3377089 (10 pages) | Cited 2 times

Online Publication Date: 8 April 2010

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We perform histogram-reweighting grand canonical Monte Carlo simulations of the Lennard-Jones fluid confined between two parallel hard walls and determine the vapor-liquid critical and coexistence properties in the range of σ ≤ H ≤ 6σ and 10σ ≤ Lx,Ly ≤ 28σ, where H is the wall separation, Lx = Ly is the system size and σ is the characteristic length. By matching the probability distribution of the ordering operator, P(M), to the three-dimensional (3D) and two-dimensional (2D) Ising universality classes according to the mixed-field finite-size scaling approach, we establish a “phase diagram” in the (H,L) plane, showing the boundary between four types of behavior: 3D, quasi-3D, quasi-2D, and 2D. In order to facilitate 2D critical point calculation, we present a four-parameter analytical expression for the 2D Ising universal distribution. We show that the infinite-system-size critical points obtained by extrapolation from the apparent 3D and 2D critical points have only minor differences with each other. In agreement with recent reports in the literature [ Jana et al., J. Chem. Phys. 130, 214707 (2009) ], we find departure from linearity in the relationship between critical temperature and inverse wall separation, as well as nonmonotonic dependence of the critical density and the liquid density at coexistence upon wall separation. Additional studies of the ST2 model of water show similar behavior, which suggests that these are quite general properties of confined fluids.
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64.70.F- Liquid-vapor transitions
05.50.+q Lattice theory and statistics (Ising, Potts, etc.)
64.60.Ht Dynamic critical phenomena
61.20.Ja Computer simulation of liquid structure

Efficient ab initio path integral hybrid Monte Carlo based on the fourth-order Trotter expansion: Application to fluoride ion-water cluster

Kimichi Suzuki, Masanori Tachikawa, and Motoyuki Shiga

J. Chem. Phys. 132, 144108 (2010); http://dx.doi.org/10.1063/1.3367724 (7 pages) | Cited 6 times

Online Publication Date: 13 April 2010

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We propose an efficient path integral hybrid Monte Carlo (PIHMC) method based on fourth-order Trotter expansion. Here, the second-order effective force is employed to generate short trial trajectories to avoid computationally expensive Hessian matrix, while the final acceptance is judged based on fourth-order effective potential. The computational performance of our PIHMC scheme is compared with that of conventional PIHMC and PIMD methods based on second- and fourth-order Trotter expansions. Our method is applied to on-the-fly ab initio PIHMC calculation of fluoride ion-water complexes, F−(H2O) and F−(D2O), at ambient temperature, particularly focusing on the geometrical isotope effect.
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31.15.A- Ab initio calculations
36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Dj Interatomic distances and angles
33.15.Fm Bond strengths, dissociation energies
31.30.Gs Hyperfine interactions and isotope effects

Replica exchange simulation method using temperature and solvent viscosity

Phuong H. Nguyen

J. Chem. Phys. 132, 144109 (2010); http://dx.doi.org/10.1063/1.3369626 (10 pages) | Cited 1 time

Online Publication Date: 14 April 2010

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We propose an efficient and simple method for fast conformational sampling by introducing the solvent viscosity as a parameter to the conventional temperature replica exchange molecular dynamics (T-REMD) simulation method. The method, named V-REMD (V stands for viscosity), uses both low solvent viscosity and high temperature to enhance sampling for each replica; therefore it requires fewer replicas than the T-REMD method. To reduce the solvent viscosity by a factor of λ in a molecular dynamics simulation, one can simply reduce the mass of solvent molecules by a factor of λ2. This makes the method as simple as the conventional method. Moreover, thermodynamic and conformational properties of structures in replicas are still useful as long as one has sufficiently sampled the Boltzmann ensemble. The advantage of the present method has been demonstrated with the simulations of the trialanine, deca-alanine, and a 16-residue β-hairpin peptides. It shows that the method could reduce the number of replicas by a factor of 1.5 to 2 as compared with the T-REMD method.
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66.20.-d Viscosity of liquids; diffusive momentum transport
02.70.Ns Molecular dynamics and particle methods
65.20.De General theory of thermodynamic properties of liquids, including computer simulation
87.14.E- Proteins
82.20.Yn Solvent effects on reactivity

Analytic gradients for Mukherjee’s multireference coupled-cluster method using two-configurational self-consistent-field orbitals

Thomas-C. Jagau, Eric Prochnow, Francesco A. Evangelista, and Jürgen Gauss

J. Chem. Phys. 132, 144110 (2010); http://dx.doi.org/10.1063/1.3370847 (9 pages) | Cited 11 times

Online Publication Date: 14 April 2010

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Analytic gradients for the state-specific multireference coupled-cluster method suggested by Mahapatra et al. [Mol. Phys. 94, 157 (1998)] (Mk-MRCC) are reported within the singles and doubles approximation using two-configurational self-consistent field (TCSCF) orbitals. The present implementation extends our previous work on Mk-MRCC gradients [ E. Prochnow et al., J. Chem. Phys. 131, 064109 (2009) ] which is based on restricted Hartree–Fock orbitals and consequently the main focus of the present paper is on the treatment of orbital relaxation at the TCSCF level using coupled-perturbed TCSCF theory. Geometry optimizations on m-arynes and nitrenes are presented to illustrate the influence of the orbitals on the computed equilibrium structures. The results are compared to those obtained at the single-reference coupled-cluster singles and doubles and at the Mk-MRCC singles and doubles level of theory when using restricted Hartree–Fock orbitals.
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31.15.bw Coupled-cluster theory
31.15.xr Self-consistent-field methods
33.15.Bh General molecular conformation and symmetry; stereochemistry

The resolution of the identity approximation for calculations of spin-spin contribution to zero-field splitting parameters

Dmitry Ganyushin, Natalie Gilka, Peter R. Taylor, Christel M. Marian, and Frank Neese

J. Chem. Phys. 132, 144111 (2010); http://dx.doi.org/10.1063/1.3367718 (11 pages) | Cited 1 time

Online Publication Date: 14 April 2010

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In this work, the resolution of the identity (RI) approximation is developed for the calculation of the electron-electron spin-spin coupling (SSC) interaction that is a central component of the zero-field splitting (ZFS) term in the effective spin Hamiltonian. The approximated integrals are then used in large-scale multireference configuration interaction treatments of the SSC interaction. The SSC contribution to the ZFS is treated using the Breit–Pauli spin-spin Hamiltonian in conjunction with first-order perturbation theory. Test calculations on a set of diatomic molecules reveal that the error of the RI approximation does not exceed 0.01 cm−1 even if standard auxiliary basis sets are used. This error of less than 1% is considered to be negligible compared to the presently achievable accuracy of the SSC calculations relative to experimental data. The present development allows the correlated ab initio calculation of ZFS parameters of larger systems such as linear polyenes and linear polyacenes. The basis set convergence of the calculated ZFS values was investigated, and the effect of electronic correlation on the calculated ZFS parameters is discussed.
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33.15.Pw Fine and hyperfine structure
31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
31.15.V- Electron correlation calculations for atoms, ions and molecules
31.15.xp Perturbation theory
33.25.+k Nuclear resonance and relaxation
31.15.aj Relativistic corrections, spin-orbit effects, fine structure; hyperfine structure
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

CH3CH2OD/D2O binary condensation in a supersonic Laval nozzle: Presence of small clusters inferred from a macroscopic energy balance

Shinobu Tanimura, Barbara E. Wyslouzil, and Gerald Wilemski

J. Chem. Phys. 132, 144301 (2010); http://dx.doi.org/10.1063/1.3360304 (22 pages) | Cited 4 times

Online Publication Date: 8 April 2010

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We determined the heat released in the condensing flow of a CH3CH2OD/D2O/carrier gas mixture (EtOD/D2O for brevity) through a supersonic Laval nozzle by integrating the equations for supersonic flow with condensation, using the static pressure, temperature, and mole fractions of EtOD and D2O monomers [ S. Tanimura, B. E. Wyslouzil, M. S. Zahniser, et al., J. Chem. Phys. 127, 034305 (2007) ] as inputs. By considering the depletion of the monomer species, the deviation of the pressure from the isentropic value, and the heat released, we estimated that ∼ 10% of the EtOD molecules are present as pure clusters (dimer to tetramer) upstream of the onset point of condensation. In contrast, clustering was not detected when only pure EtOD was present under the same conditions (temperature and the partial pressure of EtOD) for which clustering was observed in the EtOD/D2O flow. This suggests that the formation of EtOD clusters is facilitated by D2O in the EtOD/D2O flow. A comparison of the heat released to the flow and the expected heat of dissociation of the EtOD/D2O droplets suggests that small EtOD clusters persist downstream of the onset point. Both upstream and downstream of the onset point of condensation, the concentration of these clusters in the nozzle is higher than that expected at equilibrium. A possible mechanism for the overabundance of pure EtOD clusters is that they form in the mixed EtOD/D2O particles (droplets or clusters) and evaporate from them.
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47.40.Ki Supersonic and hypersonic flows
47.55.D- Drops and bubbles
47.70.Fw Chemically reactive flows
47.60.Kz Flows and jets through nozzles

Tracing molecular electronic excitation dynamics in real time and space

Anthony D. Dutoi, Lorenz S. Cederbaum, Michael Wormit, Jan Hendrik Starcke, and Andreas Dreuw

J. Chem. Phys. 132, 144302 (2010); http://dx.doi.org/10.1063/1.3353161 (18 pages) | Cited 4 times

Online Publication Date: 9 April 2010

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We present a method for studying the movement of electrons and energy within and between electronically excited molecules. The dynamically changing state is a many-electron wavepacket, for which we numerically integrate the Schrödinger equation using the ADC(2) effective Hamiltonian for the particle-hole propagator. We develop the tools necessary for following the separate motions of the particles and holes. Total particle and hole densities can be used to give an overview of the dynamics, which can be atomically decomposed in a Mulliken fashion, or individual particle and hole states give a more detailed look at the structure of an excitation. We apply our model to a neon chain, as an illustrative example, projecting an excited eigenstate of an isolated atom onto the coupled system as the initial state. In addition to demonstrating our propagation and analysis machinery, the results show a dramatic difference in excitation-energy transfer rates as a consequence of initial polarization. Furthermore, already in a system with three constituents, an important aspect of multiple coupled systems appears, in that one absorbing system essentially shields another, changing the effective sitewise coupling parameters.
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31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations

A benchmark study of the vertical electronic spectra of the linear chain radicals C2H and C4H

Ryan C. Fortenberry, Rollin A. King, John F. Stanton, and T. Daniel Crawford

J. Chem. Phys. 132, 144303 (2010); http://dx.doi.org/10.1063/1.3376073 (10 pages) | Cited 7 times

Online Publication Date: 9 April 2010

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The ability of coupled-cluster models to predict vertical excitation energies is tested on the electronic states of carbon-chain radicals of particular relevance to interstellar chemistry. Using spin-unrestricted and -restricted reference wave functions, the coupled-cluster singles and doubles (CCSD) model and a triples-including model (CC3) are tested on the σ radicals C2H and C4H. Both molecules exhibit low-lying excited states with significant double-excitation character (as well as states of quartet multiplicity) and are thus challenging cases for excited-state approaches. In addition, we employ two diagnostics for the reliability of the CC results: the approximate excitation level (AEL) relative to the ground state and the difference between excitation energies obtained with spin-unrestricted and spin-restricted reference wave functions (the U-R difference). We find that CCSD yields poor excitation energies for states with AEL significantly larger than ca. 1.1 and/or large U-R differences, as well as for certain states exhibiting large spin contamination or other inadequacies in the reference determinant. In such cases, connected triple excitations can be included in the model and generally provide improved results. Furthermore, we find that large discrepancies exist between CC and multireference (MR) results for certain states. These disagreements are not related to basis-set effects, but likely arise from the lack of spin adaptation in conventional spin-orbital CC implementations and active space selection in the MR models.
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31.15.bw Coupled-cluster theory
31.15.vq Electron correlation calculations for polyatomic molecules

Quasiclassical dynamics and kinetics of the N+NO→N2+O, NO+N atmospheric reactions

Pablo Gamallo, Rodrigo Martínez, R. Sayós, and Miguel González

J. Chem. Phys. 132, 144304 (2010); http://dx.doi.org/10.1063/1.3364867 (9 pages) | Cited 2 times

Online Publication Date: 9 April 2010

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The kinetics and dynamics of the title reactions were studied using the quasiclassical trajectory (QCT) method and two ab initio analytical potential energy surfaces (PESs) developed by our group. In addition to the rate constant (T: 10–5000 K), we also considered a broad set of dynamic properties as a function of collision energy (up to 1.0 eV) and the rovibrational state of NO (v = 0–2,j = 1,8,12). The production of N2+O, reaction ( 1 ), dominates the reactivity of the N+NO system over the conditions studied, as expected from the large energy barriers associated to the NO+N exchange reaction, reaction ( 2 ). Moreover, the ground PES, which is barrierless for reaction ( 1 ), plays a dominant role. Most of the results were interpreted according to the properties of the PESs involved and the kinematics of the system. The QCT rate constants of reaction ( 1 ) are in agreement with the experimental data (T: 47–3500 K), including very recent low temperature measurements, and also with variational transition state kinetics and most of quantum dynamics calculations. In addition, the QCT average vibrational energy content of the N2 product also agrees with the experimental and quantum data. The PESs used here could also be useful to determine equilibrium and nonequilibrium reaction rates at very high temperatures (e.g., 5000–15 000 K).
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.33.Tb Atmospheric chemistry
82.20.Fd Collision theories; trajectory models
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Ln Semiclassical theory of reactions and/or energy transfer

Experimental and theoretical study of the absorption properties of thiolated diamondoids

Lasse Landt, Christoph Bostedt, David Wolter, Thomas Möller, Jeremy E. P. Dahl, Robert M. K. Carlson, Boryslav A. Tkachenko, Andrey A. Fokin, Peter R. Schreiner, Alexander Kulesza, Roland Mitrić, and Vlasta Bonačić-Koutecký

J. Chem. Phys. 132, 144305 (2010); http://dx.doi.org/10.1063/1.3356034 (6 pages)

Online Publication Date: 13 April 2010

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Nanoscale hybrid systems are a new class of molecular aggregates that offer numerous new possibilities in materials design. Diamondoid thiols are promising nanoscale building blocks for such hybrid systems. They allow the incorporation of functional groups and the investigation of their effects on the unique materials’ properties of diamondoids. Here we combine experimental data with ab initio theory to explore the optical properties of diamondoid thiols and their dependence on size and shape. Agreement between theoretically and experimentally obtained absorption spectra allows the identification of the nature of the optical transitions that are responsible for some photophysical and photochemical processes. We show that the optical properties of diamondoid thiols in the deep UV regime depend on the functionalization site but are largely size independent. Our findings provide an explanation for the disappearance of diamondoid UV photoluminescence upon thiolation for smaller diamondoids. However, our theoretical results indicate that for larger diamondoid thiols beyond the critical size of six diamondoid cages the lowest energy transitions are characterized by diamondoidlike states suggesting that UV luminescence may be regained.
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78.66.Qn Polymers; organic compounds
31.50.Df Potential energy surfaces for excited electronic states
31.15.bw Coupled-cluster theory
82.50.-m Photochemistry
31.15.A- Ab initio calculations
78.55.Kz Solid organic materials
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Prewetting transitions of one site associating fluids

Sandip Khan and Jayant K. Singh

J. Chem. Phys. 132, 144501 (2010); http://dx.doi.org/10.1063/1.3382345 (8 pages) | Cited 1 time

Online Publication Date: 8 April 2010

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Prewetting transitions are studied for Lennard-Jones (LJ) based dimer forming associating fluids, on a structureless surface represented by LJ 9–3 type potential, for various association strengths using grand-canonical transition matrix Monte Carlo (GC-TMMC) and histogram reweighting techniques. Occurrences of prewetting transition are observed for association strengths: εaf = 2.0, 4.0, 6.0, 8.0, and 10.0. Structural properties, monomer fraction, and orientation order profile of thin-thick film of one-site associating fluids are presented. Wetting temperature, Tw, and prewetting critical temperature, Tpwc, increases with increasing association strength, which is in agreement with the results of the density functional theory (DFT). Length of prewetting line, on the other hand, is found to decrease first with increasing association energy until εaf = 8.0 and subsequently found to increase substantially for εaf = 10. This behavior is contrary to the prediction from the DFT. We observe that the boundary tension of thin-thick film via GC-TMMC and finite size scaling exhibits a maximum with respect to association strength.
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61.20.Qg Structure of associated liquids: electrolytes, molten salts, etc.
61.20.Ja Computer simulation of liquid structure
68.15.+e Liquid thin films
68.18.Jk Phase transitions in liquid thin films
68.18.Fg Liquid thin film structure: measurements and simulations

Positive and negative cluster ions from liquid ethanol by fast ion bombardment

M. Kaneda, M. Shimizu, T. Hayakawa, Y. Iriki, H. Tsuchida, and A. Itoh

J. Chem. Phys. 132, 144502 (2010); http://dx.doi.org/10.1063/1.3367767 (6 pages)

Online Publication Date: 8 April 2010

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Secondary ion mass spectra have been measured for the first time for a liquid ethanol target bombarded by 2.0 MeV He+ ions. Positive and negative ion spectra exhibit evidently a series of cluster ions of the forms [(EtOH)nH]+ and [(EtOH)n−H]−, respectively, in addition to light fragment ions from intact parent molecules. It was found that these cluster ions are produced only from liquid phase ethanol. Both positive and negative secondary ion spectra show similar cluster size distributions with almost the same decay slope. We also present for the first time the cluster ion distribution emitted from the liquid at different liquid temperatures.
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34.35.+a Interactions of atoms and molecules with surfaces
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)

Unique diffusion behavior observed in supercritical ethanol

Swapan K. Ghosh and Kaoru Tsujii

J. Chem. Phys. 132, 144503 (2010); http://dx.doi.org/10.1063/1.3373404 (10 pages)

Online Publication Date: 9 April 2010

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We have systematically investigated the diffusion behavior of silica nanoparticles within supercritical ethanol, in terms of solvent properties by varying temperature (T) and pressure (P), to elucidate how the inhomogeneous solvent structures and density fluctuations in the solvent affect the diffusion behavior of solute particles. Results show that at a constant pressure, the diffusion coefficient (D) of the particles increases with increasing temperature, reaches the maximum (Dmax) within the gaslike supercritical fluid (slightly below the ridge), and finally decreases abruptly at very low fluid density when temperature is increased further. Results reveal that D is appreciably larger than the theoretical prediction (Einstein–Stokes relationship) in the vicinity of the critical density (ρc) of the solvent. We interestingly observed that D becomes maximum (Dmax) at a particular thermodynamic condition (Ti,Pi), which is expressed by the empirical formula Tri = Pri0.16 (for Tri>1, Pri>1). Here, Tri = Ti/Tc and Pri = Pi/Pc; Tc and Pc are the temperature and the pressure at critical point, respectively. Results further reveal that Dmax increases significantly with decreasing solvent density within the gaslike supercritical fluid where the changes in viscosities are negligible. These findings are unique, novel, and intriguing. We suggest that the enhancement of the diffusion coefficient in the vicinity of the critical density and the abrupt decrease in the diffusion coefficient in very low density gaslike fluid are associated with the change in the solvent-solvent and solute-solvent direct correlation function (related to the effective interaction potential) upon density change when the fluid crosses the ridge of density fluctuations and within the gaslike fluid.
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66.10.C- Diffusion and thermal diffusion
64.75.Xc Phase separation and segregation in colloidal systems
82.70.Dd Colloids

Dispersion forces between solvated electrons

Gennady N. Chuev

J. Chem. Phys. 132, 144504 (2010); http://dx.doi.org/10.1063/1.3382542 (11 pages)

Online Publication Date: 12 April 2010

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Using the path integral centroid approach, we investigate dispersion interactions between electrons solvated in metal-ammonia solutions. We have argued that at finite metal concentrations, the behavior of the solvated electrons is controlled by these interactions. The latter result in a peculiar nonmetal-metal transition, which appears as a sharp dielectric enhancement and a mechanical instability of the system. Our results indicate also that the solvated electrons are to be considered as a two-component mixture consisting of localized and delocalized electrons beyond the critical density corresponding to this mechanical instability.
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34.20.Gj Intermolecular and atom-molecule potentials and forces

Insights on the origin of the Debye process in monoalcohols from dielectric spectroscopy under extreme pressure conditions

D. Fragiadakis, C. M. Roland, and R. Casalini

J. Chem. Phys. 132, 144505 (2010); http://dx.doi.org/10.1063/1.3374820 (9 pages) | Cited 9 times

Online Publication Date: 12 April 2010

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The dielectric spectra of most simple liquids are characterized by two relaxation processes: (i) the α-process, an intense, broad non-Debye relaxation with a non-Arrhenius temperature dependence and (ii) a β process, evident mainly below the glass transition and having nearly Arrhenius temperature behavior. However, the dielectric spectra of monoalcohols show three processes: two that resemble those of normal liquids and a third very intense Debye peak at lower frequencies, which is non-Arrhenius. Interestingly, this third process is not observed with other techniques such as light scattering and mechanical spectroscopy. There is a disagreement in the literature concerning the nature of this third relaxation. We investigated 2-ethyl-1-hexanol under high pressures (up to ∼ 1.4 GPa) over a broad range of temperatures. The Debye process, which is the slowest, is strongly affected by pressure. At higher pressures the relaxation times and intensities of the two non-Arrhenius relaxations become more nearly equal. In light of these results, we propose a modified interpretation of the relaxation processes and their underlying structures in monoalcohols.
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77.84.Nh Liquids, emulsions, and suspensions; liquid crystals
77.84.Jd Polymers; organic compounds
77.22.Gm Dielectric loss and relaxation
64.70.pm Liquids
62.50.-p High-pressure effects in solids and liquids

Heterogeneous crystallization of hard spheres on patterned substrates

Wen-Sheng Xu, Zhao-Yan Sun, and Li-Jia An

J. Chem. Phys. 132, 144506 (2010); http://dx.doi.org/10.1063/1.3383239 (7 pages) | Cited 7 times

Online Publication Date: 12 April 2010

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We report a numerical investigation of the crystallization of monodisperse hard spheres on different patterned substrates. We find that the duration of a metastable fluid state, which may last for relatively long time in the homogeneous crystallization, can be deeply reduced or almost eliminated when a substrate is used as the seed of crystallization. In the presence of the square patterned substrate, which has the basic character of the body-centered cubic (bcc) crystal structure, a transient bcc crystal phase was observed, suggesting that the bcc structure could be possible to be stabilized by the square patterned substrates. The process of crystallization becomes complicated when the patterned substrate is incommensurate with the bulk crystal. Furthermore, a purely face-centered cubic or hexagonal close packed crystal phase with stacking faults can be generated by using a certain patterned substrate without other factors such as gravity.
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64.70.dg Crystallization of specific substances
61.72.Nn Stacking faults and other planar or extended defects

Levitation effect in zeolites: Quasielastic neutron scattering and molecular dynamics study of pentane isomers in zeolite NaY

Bhaskar J. Borah, H. Jobic, and S. Yashonath

J. Chem. Phys. 132, 144507 (2010); http://dx.doi.org/10.1063/1.3367894 (11 pages) | Cited 3 times

Online Publication Date: 12 April 2010

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We report the quasielastic neutron scattering (QENS) and molecular dynamics (MD) investigations into diffusion of pentane isomers in zeolite NaY. The molecular cross section perpendicular to the long molecular axis varies for the three isomers while the mass and the isomer-zeolite interaction remains essentially unchanged. Both QENS and MD results show that the branched isomers neopentane and isopentane have higher self-diffusivities as compared with n-pentane at 300 K in NaY zeolite. This result provides direct experimental evidence for the existence of nonmonotonic, anomalous dependence of self-diffusivity on molecular diameter known as the levitation effect. The energetic barrier at the bottleneck derived from MD simulations exists for n-pentane which lies in the linear regime while no such barrier is seen for neopentane which is located clearly in the anomalous regime. Activation energy is in the order Ea(n-pentane)>Ea(isopentane)>Ea(neopentane) consistent with the predictions of the levitation effect. In the liquid phase, it is seen that D(n-pentane)>D(isopentane)>D(neopentane) and Ea(n-pentane)<Ea(isopentane)<Ea(neopentane). Intermediate scattering function for small wavenumbers obtained from MD follows a single exponential decay for neopentane and isopentane. For n-pentane, a single exponential fit provides a poor fit especially at short times. Cage residence time is largest for n-pentane and lowest for neopentane. For neopentane, the width of the self-part of the dynamic structure factor shows a near monotonic decrease with wavenumber. For n-pentane a minimum is seen near k = 0.5 Å−1 suggesting a slowing down of motion around the 12-ring window, the bottleneck for diffusion. Finally, the result that the branched isomer has a higher diffusivity as compared with the linear analog is at variation from what is normally seen.
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61.20.Ja Computer simulation of liquid structure
66.10.C- Diffusion and thermal diffusion
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