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28 May 2012

Volume 136, Issue 20 (partial)

Issue Cover Spotlight Figure

J. Chem. Phys. 136, 204301 (2012); http://dx.doi.org/10.1063/1.4714347 (9 pages)

Peter Botschwina and Rainer Oswald
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back to top Theoretical Methods and Algorithms

Many-body calculations of low-energy eigenstates in magnetic and periodic systems with self-healing diffusion Monte Carlo: Steps beyond the fixed phase

Fernando Agustín Reboredo

J. Chem. Phys. 136, 204101 (2012); http://dx.doi.org/10.1063/1.4711023 (16 pages)

Online Publication Date: 22 May 2012

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The self-healing diffusion Monte Carlo algorithm (SHDMC) [F. A. Reboredo, R. Q. Hood, and P. R. C. Kent, Phys. Rev. B 79, 195117 (2009);10.1103/PhysRevB.79.195117 F. A. Reboredo, Phys. Rev. B 80, 125110 (2009)10.1103/PhysRevB.80.125110] is extended to study the ground and excited states of magnetic and periodic systems. The method converges to exact eigenstates as the statistical data collected increase if the wave function is sufficiently flexible. It is shown that the dimensionality of the nodal surface is dependent on whether phase is a scalar function or not. A recursive optimization algorithm is derived from the time evolution of the mixed probability density, which is given by an ensemble of electronic configurations (walkers) with complex weight. This complex weight allows the phase of the fixed-node wave function to move away from the trial wave function phase. This novel approach is both a generalization of SHDMC and the fixed-phase approximation [G. Ortiz, D. M. Ceperley, and R. M. Martin, Phys Rev. Lett. 71, 2777 (1993)10.1103/PhysRevLett.71.2777]. When used recursively it simultaneously improves the node and the phase. The algorithm is demonstrated to converge to nearly exact solutions of model systems with periodic boundary conditions or applied magnetic fields. The computational cost is proportional to the number of independent degrees of freedom of the phase. The method is applied to obtain low-energy excitations of Hamiltonians with magnetic field. Periodic boundary conditions are also considered optimizing wave functions with twisted boundary conditions which are included in a many-body Bloch phase. The potential applications of this new method to study periodic, magnetic, and complex Hamiltonians are discussed.
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02.70.Ss Quantum Monte Carlo methods
02.10.Ud Linear algebra
02.50.-r Probability theory, stochastic processes, and statistics

Phase diagram and universality of the Lennard-Jones gas-liquid system

Hiroshi Watanabe, Nobuyasu Ito, and Chin-Kun Hu

J. Chem. Phys. 136, 204102 (2012); http://dx.doi.org/10.1063/1.4720089 (7 pages)

Online Publication Date: 23 May 2012

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The gas-liquid phase transition of the three-dimensional Lennard-Jones particles system is studied by molecular dynamics simulations. The gas and liquid densities in the coexisting state are determined with high accuracy. The critical point is determined by the block density analysis of the Binder parameter with the aid of the law of rectilinear diameter. From the critical behavior of the gas-liquid coexisting density, the critical exponent of the order parameter is estimated to be β = 0.3285(7). Surface tension is estimated from interface broadening behavior due to capillary waves. From the critical behavior of the surface tension, the critical exponent of the correlation length is estimated to be ν = 0.63(4). The obtained values of β and ν are consistent with those of the Ising universality class.
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61.20.Ja Computer simulation of liquid structure
68.03.Cd Surface tension and related phenomena
64.60.F- Equilibrium properties near critical points, critical exponents
64.70.F- Liquid-vapor transitions

General formulation of spin-flip time-dependent density functional theory using non-collinear kernels: Theory, implementation, and benchmarks

Yves A. Bernard, Yihan Shao, and Anna I. Krylov

J. Chem. Phys. 136, 204103 (2012); http://dx.doi.org/10.1063/1.4714499 (17 pages)

Online Publication Date: 23 May 2012

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We report an implementation of the spin-flip (SF) variant of time-dependent density functional theory (TD-DFT) within the Tamm-Dancoff approximation and non-collinear (NC) formalism for local, generalized gradient approximation, hybrid, and range-separated functionals. The performance of different functionals is evaluated by extensive benchmark calculations of energy gaps in a variety of diradicals and open-shell atoms. The benchmark set consists of 41 energy gaps. A consistently good performance is observed for the Perdew-Burke-Ernzerhof (PBE) family, in particular PBE0 and PBE50, which yield mean average deviations of 0.126 and 0.090 eV, respectively. In most cases, the performance of original (collinear) SF-TDDFT with 50-50 functional is also satisfactory (as compared to non-collinear variants), except for the same-center diradicals where both collinear and non-collinear SF variants that use LYP or B97 exhibit large errors. The accuracy of NC-SF-TDDFT and collinear SF-TDDFT with 50-50 and BHHLYP is very similar. Using PBE50 within collinear formalism does not improve the accuracy.
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31.15.ee Time-dependent density functional theory

Sensitivity of the properties of ruthenium “blue dimer” to method, basis set, and continuum model

Abdullah Ozkanlar and Aurora E. Clark

J. Chem. Phys. 136, 204104 (2012); http://dx.doi.org/10.1063/1.4719937 (7 pages)

Online Publication Date: 23 May 2012

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The ruthenium “blue dimer” [(bpy)2RuIIIOH2]2O4+ is best known as the first well-defined molecular catalyst for water oxidation. It has been subject to numerous computational studies primarily employing density functional theory. However, those studies have been limited in the functionals, basis sets, and continuum models employed. The controversy in the calculated electronic structure and the reaction energetics of this catalyst highlights the necessity of benchmark calculations that explore the role of density functionals, basis sets, and continuum models upon the essential features of blue-dimer reactivity. In this paper, we report Kohn-Sham complete basis set (KS-CBS) limit extrapolations of the electronic structure of “blue dimer” using GGA (BPW91 and BP86), hybrid-GGA (B3LYP), and meta-GGA (M06-L) density functionals. The dependence of solvation free energy corrections on the different cavity types (UFF, UA0, UAHF, UAKS, Bondi, and Pauling) within polarizable and conductor-like polarizable continuum model has also been investigated. The most common basis sets of double-zeta quality are shown to yield results close to the KS-CBS limit; however, large variations are observed in the reaction energetics as a function of density functional and continuum cavity model employed.
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31.15.E- Density-functional theory
82.30.Nr Association, addition, insertion, cluster formation

Local explicitly correlated second- and third-order Møller–Plesset perturbation theory with pair natural orbitals

Christof Hättig, David P. Tew, and Benjamin Helmich

J. Chem. Phys. 136, 204105 (2012); http://dx.doi.org/10.1063/1.4719981 (14 pages)

Online Publication Date: 24 May 2012

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We present an algorithm for computing explicitly correlated second- and third-order Møller–Plesset energies near the basis set limit for large molecules with a cost that scales formally as N4 with system size N. This is achieved through a hybrid approach where locality is exploited first through orbital specific virtuals (OSVs) and subsequently through pair natural orbitals (PNOs) and integrals are approximated using density fitting. Our method combines the low orbital transformation costs of the OSVs with the compactness of the PNO representation of the doubles amplitude vector. The N4 scaling does not rely upon the a priori definition of domains, enforced truncation of pair lists, or even screening and the energies converge smoothly to the canonical values with decreasing occupation number thresholds, used in the selection of the PNO basis. For MP2.5 intermolecular interaction energies, we find that 99% of benchmark basis set limit correlation energy contributions are recovered using an aug-cc-pVTZ basis and that on average only 50 PNOs are required to correlate the significant orbital pairs.
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31.15.bw Coupled-cluster theory
31.15.xp Perturbation theory
34.20.Gj Intermolecular and atom-molecule potentials and forces

Diffusion in the presence of cylindrical obstacles arranged in a square lattice analyzed with generalized Fick-Jacobs equation

Leonardo Dagdug, Marco-Vinicio Vazquez, Alexander M. Berezhkovskii, Vladimir Yu. Zitserman, and Sergey M. Bezrukov

J. Chem. Phys. 136, 204106 (2012); http://dx.doi.org/10.1063/1.4720385 (5 pages)

Online Publication Date: 24 May 2012

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The generalized Fick-Jacobs equation is widely used to study diffusion of Brownian particles in three-dimensional tubes and quasi-two-dimensional channels of varying constraint geometry. We show how this equation can be applied to study the slowdown of unconstrained diffusion in the presence of obstacles. Specifically, we study diffusion of a point Brownian particle in the presence of identical cylindrical obstacles arranged in a square lattice. The focus is on the effective diffusion coefficient of the particle in the plane perpendicular to the cylinder axes, as a function of the cylinder radii. As radii vary from zero to one half of the lattice period, the effective diffusion coefficient decreases from its value in the obstacle free space to zero. Using different versions of the generalized Fick-Jacobs equation, we derive simple approximate formulas, which give the effective diffusion coefficient as a function of the cylinder radii, and compare their predictions with the values of the effective diffusion coefficient obtained from Brownian dynamics simulations. We find that both Reguera-Rubi and Kalinay-Percus versions of the generalized Fick-Jacobs equation lead to quite accurate predictions of the effective diffusion coefficient (with maximum relative errors below 4% and 7%, respectively) over the entire range of the cylinder radii from zero to one half of the lattice period.
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05.60.-k Transport processes
05.50.+q Lattice theory and statistics (Ising, Potts, etc.)
05.40.Jc Brownian motion
02.40.-k Geometry, differential geometry, and topology
02.30.Mv Approximations and expansions

Perturbative treatment of triple excitations in internally contracted multireference coupled cluster theory

Matthias Hanauer and Andreas Köhn

J. Chem. Phys. 136, 204107 (2012); http://dx.doi.org/10.1063/1.4718700 (18 pages)

Online Publication Date: 24 May 2012

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Internally contracted multireference coupled cluster (ic-MRCC) methods with perturbative treatment of triple excitations are formulated based on Dyall's definition of a zeroth-order Hamiltonian. The iterative models ic-MRCCSDT-1, ic-MRCC3, and their variants ic-MRCCSD(T), ic-MRCC(3) which determine the energy correction from triples by a non-iterative step are consistent in the single-reference limit with CCSDT-1a, CC3, CCSD(T), and CC(3), respectively. Numerical tests on the potential energy surfaces of BeH2, H2O, and N2 as well as on the structure and harmonic vibrational frequencies of the ozone molecule show that these methods account very well for higher order correlation effects. The ic-MRCCSD(T) method is further applied to the geometry optimization and harmonic frequencies of the symmetric vibrational modes of the binuclear transition metal oxide Ni2O2, to the singlet-triplet splittings of o-, m-, and p-benzyne and to a ring-opening reaction of an azirine compound with the molecular formula C6H7NO. The size of the active spaces used in this study ranges from CAS(2,2) to CAS(8,8). Comparisons of results based on differently sized active spaces indicate that the ic-MRCCSD(T) method provides a highly accurate and efficient treatment of both static and dynamic electron correlation in connection with minimal active spaces.
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31.15.bw Coupled-cluster theory
31.15.eg Exchange-correlation functionals (in current density functional theory)
31.15.xp Perturbation theory
31.50.Df Potential energy surfaces for excited electronic states
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations

A sequential transformation approach to the internally contracted multireference coupled cluster method

Francesco A. Evangelista, Matthias Hanauer, Andreas Köhn, and Jürgen Gauss

J. Chem. Phys. 136, 204108 (2012); http://dx.doi.org/10.1063/1.4718704 (11 pages)

Online Publication Date: 24 May 2012

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The internally contracted multireference coupled cluster (ic-MRCC) approach is formulated using a new wave function ansatz based on a sequential transformation of the reference function (sqic-MRCC). This alternative wave function simplifies the formulation of computationally viable methods while preserving the accuracy of the ic-MRCC approach. The structure of the sqic-MRCC wave function allows folding the effect of the single excitations into a similarity-transformed Hamiltonian whose particle rank is equal to the one of the Hamiltonian. Consequently, we formulate an approximation to the sqic-MRCC method with singles and doubles (included respectively up to fourfold and twofold commutators, sqic-MRCCSD[2]) that contains all terms present in the corresponding single-reference coupled cluster scheme. Computations of the potential energy curves for the dissociation of BeH2 show that the untruncated sqic-MRCCSD scheme yields results that are almost indistinguishable from the ordinary ic-MRCCSD method. The energy obtained from the computationally less expensive sqic-MRCCSD[2] approximation is found to deviate from the full ic-MRCCSD method by less than 0.2 mEh for BeH2, while, in the case of water, the harmonic vibrational frequencies of ozone, the singlet-triplet splitting of p-benzyne, and the dissociation curve of N2, sqic-MRCCSD[2] faithfully reproduces the results obtained via the ic-MRCCSD scheme truncated to two commutators. A formal proof is given of the equivalence of the ic-MRCC and sqic-MRCC methods with the internally contracted and full configuration interaction approaches.
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31.15.bw Coupled-cluster theory
31.50.Df Potential energy surfaces for excited electronic states
33.20.Tp Vibrational analysis
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Dispersion-free component of non-covalent interaction via mutual polarization of fragment densities

Marcin Modrzejewski, Łukasz Rajchel, Małgorzata M. Szczęśniak, and Grzegorz Chałasiński

J. Chem. Phys. 136, 204109 (2012); http://dx.doi.org/10.1063/1.4705282 (9 pages)

Online Publication Date: 24 May 2012

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Comprehensive tests within a diverse set of noncovalently bonded systems are carried out to assess the performance of the recently-developed dispersion-free approach in the framework of density functional theory [Ł. Rajchel, P. Żuchowski, M. Szczęśniak, and G. Chałasiński, Phys. Rev. Lett. 104, 163001 (2010)]10.1103/PhysRevLett.104.163001. A numerical algorithm which cures the convergence problems of the previous implementation is presented.
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34.20.Gj Intermolecular and atom-molecule potentials and forces
31.15.E- Density-functional theory

Explicitly correlated second-order Møller-Plesset perturbation theory employing pseudospectral numerical quadratures

Denis Bokhan and Dmitrii N. Trubnikov

J. Chem. Phys. 136, 204110 (2012); http://dx.doi.org/10.1063/1.4719037 (9 pages)

Online Publication Date: 24 May 2012

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We implemented explicitly correlated second-order Møller-Plesset perturbation theory with numerical quadratures using pseudospectral construction of grids. Introduction of pseudospectral approach for the calculation of many-electron integrals gives a possibility to use coarse grids without significant loss of precision in correlation energies, while the number of points in the grid is reduced about nine times. The use of complementary auxiliary basis sets as the sets of dealiasing functions is justified at both theoretical and computational levels. Benchmark calculations for a set of 16 molecules have shown the possibility to keep an error of second-order correlation energies within 1 milihartree (mH) with respect to MP2-F12 method with dense grids. Numerical tests for a set of 13 isogyric reactions are also performed.
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31.15.xp Perturbation theory
31.15.V- Electron correlation calculations for atoms, ions and molecules

Derivative discontinuity, bandgap and lowest unoccupied molecular orbital in density functional theory

Weitao Yang, Aron J. Cohen, and Paula Mori-Sánchez

J. Chem. Phys. 136, 204111 (2012); http://dx.doi.org/10.1063/1.3702391 (13 pages)

Online Publication Date: 24 May 2012

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The conventional analysis of Perdew and Levy, and Sham and Schlüter shows that the functional derivative discontinuity of the exchange-correlation density functional plays a critical role in the correct prediction of bandgaps, or the chemical hardness. In a recent work by the present authors, explicit expressions for bandgap prediction with all common types of exchange-correlation functionals have been derived without invoking the concept of exchange-correlation energy functional derivative discontinuity at all. We here analyze the two approaches and establish their connection and difference. The present analysis further leads to several important results: (1) The lowest unoccupied molecular orbital (LUMO) in DFT has as much meaning in describing electron addition as the highest occupied molecular orbital (HOMO) in describing electron removal. (2) Every term in the total energy functional contributes to the energy gap because of the discontinuity of the derivative of the density (or density matrix) with respect to the number of electrons, ((∂ρs(r′,r))/∂N)vs, at integers. (3) Consistent with the Perdew-Levy-Sham-Schlüter conclusion that the exact Kohn-Sham energy gap differs from the fundamental bandgap by a finite correction due to the functional derivative discontinuity of the exchange-correlation energy, we show that the exchange-correlation functional cannot be an explicit and differentiable functional of the electron density, either local or nonlocal. The last result is further strengthened when we consider Mott insulators. There, the exact exchange-correlation functional needs to have an explicitly discontinuous (nondifferentiable) dependence on the density or the density matrix. (4) We obtain exact conditions on the derivatives of total energy with respect to the spin-up and spin-down number of electrons.
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31.15.eg Exchange-correlation functionals (in current density functional theory)
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
31.15.ec Hohenberg-Kohn theorem and formal mathematical properties, completeness theorems

Analytic gradient for second order Møller-Plesset perturbation theory with the polarizable continuum model based on the fragment molecular orbital method

Takeshi Nagata, Dmitri G. Fedorov, Hui Li, and Kazuo Kitaura

J. Chem. Phys. 136, 204112 (2012); http://dx.doi.org/10.1063/1.4714601 (14 pages)

Online Publication Date: 24 May 2012

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A new energy expression is proposed for the fragment molecular orbital method interfaced with the polarizable continuum model (FMO/PCM). The solvation free energy is shown to be more accurate on a set of representative polypeptides with neutral and charged residues, in comparison to the original formulation at the same level of the many-body expansion of the electrostatic potential determining the apparent surface charges. The analytic first derivative of the energy with respect to nuclear coordinates is formulated at the second-order Møller-Plesset (MP2) perturbation theory level combined with PCM, for which we derived coupled perturbed Hartree-Fock equations. The accuracy of the analytic gradient is demonstrated on test calculations in comparison to numeric gradient. Geometry optimization of the small Trp-cage protein (PDB: 1L2Y) is performed with FMO/PCM/6-31(+)G(d) at the MP2 and restricted Hartree-Fock with empirical dispersion (RHF/D). The root mean square deviations between the FMO optimized and NMR experimental structure are found to be 0.414 and 0.426 Å for RHF/D and MP2, respectively. The details of the hydrogen bond network in the Trp-cage protein are revealed.
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87.15.Pc Electronic and electrical properties
02.60.Pn Numerical optimization
87.15.Cc Folding: thermodynamics, statistical mechanics, models, and pathways
87.14.E- Proteins
87.15.B- Structure of biomolecules
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
FREE

Complexes of type C6H7+·L (L = N2 and CO2) studied by explicitly correlated coupled cluster theory

Peter Botschwina and Rainer Oswald

J. Chem. Phys. 136, 204301 (2012); http://dx.doi.org/10.1063/1.4714347 (9 pages)

Online Publication Date: 22 May 2012

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Complexes of the benzenium ion (C6H7+) with N2 or CO2 have been studied by explicitly correlated coupled cluster theory at the CCSD(T)-F12x (x = a, b) level [T. B. Adler et al., J. Chem. Phys. 127, 221106 (2007)10.1063/1.2817618] and the double-hybrid density functional B2PLYP-D [T. Schwabe and S. Grimme, Phys. Chem. Chem. Phys. 9, 3397 (2007)10.1039/b704725h]. Improved harmonic vibrational wavenumbers for C6H7+ have been obtained by CCSD(T*)-F12a calculations with the VTZ-F12 basis set. Combining them with previous B2PLYP-D anharmonic contributions we arrive at anharmonic wavenumbers which are in excellent agreement with recent experimental data from p-H2 matrix isolation IR spectroscopy [M. Bahou et al., J. Chem. Phys. 136, 154304 (2012)10.1063/1.3703502]. The energetically most favourable conformer of C6H7+·N2 shows a π-bonded structure similar to C6H7+·Rg (Rg = Ne, Ar) [P. Botschwina and R. Oswald, J. Phys. Chem. A 115, 13664 (2011)10.1021/jp207905t] with De ≈ 870 cm−1. For C6H7+·CO2, a slightly lower energy is calculated for a conformer with the CO2 ligand lying in the ring-plane of the C6H7+ moiety (De ≈ 1508 cm−1). It may be discriminated from other conformers through a strong band predicted at 1218 cm−1, red-shifted by 21 cm−1 from the corresponding band of free C6H7+.
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31.15.bw Coupled-cluster theory
31.15.E- Density-functional theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.20.Tp Vibrational analysis

Angular and energy distribution of fragment ions in dissociative double photoionization of acetylene molecules at 39 eV

M. Alagia, C. Callegari, P. Candori, S. Falcinelli, F. Pirani, R. Richter, S. Stranges, and F. Vecchiocattivi

J. Chem. Phys. 136, 204302 (2012); http://dx.doi.org/10.1063/1.4720350 (6 pages)

Online Publication Date: 22 May 2012

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The two-body dissociation reactions of the dication, C2H22+, produced by 39.0 eV double photoionization of acetylene molecules, have been studied by coupling photoelectron-photoion-photoion coincidence and ion imaging techniques. The results provide the kinetic energy and angular distributions of product ions. The analysis of the results indicates that the dissociation leading to C2H++H+ products occurs through a metastable dication with a lifetime of 108 ± 22 ns, and a kinetic energy release (KER) distribution exhibiting a maximum at ∼4.3 eV with a full width at half maximum (FWHM) of about 60%. The reaction leading to CH 2++C+ occurs in a time shorter than the typical rotational period of the acetylene molecules (of the order of 10−12 s). The KER distribution of product ions for this reaction, exhibits a maximum at ∼4.5 eV with a FWHM of about 28%. The symmetric dissociation, leading to CH+ + CH+, exhibits a KER distribution with a maximum at ∼5.2 eV with a FWHM of 44%. For the first two reactions the angular distributions of ion products also indicate that the double photoionization of acetylene occurs when the neutral molecule is mainly oriented perpendicularly to the light polarization vector.
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82.50.-m Photochemistry
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.53.Eb Pump probe studies of photodissociation

Vibrationally averaged isotropic dispersion energy coefficients of the parahydrogen dimer

Timothy C. Lillestolen and Robert J. Hinde

J. Chem. Phys. 136, 204303 (2012); http://dx.doi.org/10.1063/1.4708807 (4 pages)

Online Publication Date: 22 May 2012

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We compare the sum-over-states and coupled cluster linear response formalisms for the determination of imaginary-frequency polarizabilities of H2. Using both approaches, we compute isotropic dispersion energy coefficients Cn (n = 6, 8, 10) for H2–H2 molecular pairs over a wide range of H2 bond lengths. We present vibrationally averaged dispersion energy coefficients for H2–H2, H2–D2, and D2–D2 molecular pairs and examine the coefficients’ convergence with respect to basis set.
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31.15.vn Electron correlation calculations for diatomic molecules
31.15.bw Coupled-cluster theory
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Dj Interatomic distances and angles

Comprehensive vacuum ultraviolet photoionization study of the CF3• trifluoromethyl radical using synchrotron radiation

Héloïse Dossmann (Soldi-Lose), Gustavo A. Garcia, Laurent Nahon, Barbara K. C. de Miranda, and Christian Alcaraz

J. Chem. Phys. 136, 204304 (2012); http://dx.doi.org/10.1063/1.4719529 (8 pages)

Online Publication Date: 22 May 2012

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The trifluoromethyl radical, CF3•, is studied for the first time by means of threshold photoelectron spectroscopy (TPES). The radical is produced in the gas phase using the flash-pyrolysis technique from hexafluoroethane as a precursor. CF3+ total ion yield and mass-selected TPES of the radical are recorded using a spectrometer based upon velocity map imaging and Wiley-McLaren time-of-flight coupled to the synchrotron radiation. The high resolution of the instrument and of the photons allows the observation of rich vibrational progressions in the TPES of CF3•. By using Franck-Condon factors computed by Bowman and coworkers, we have been able to simulate the TPES. The initial vibrational temperature of the radical beam has been evaluated at 350 ± 70 K. The structures have been identified as transitions between (n1,n2) and (n1+,n2+) vibrational levels of CF3 and CF3+ with small excitation of the breathing mode, ν1+, and large excitation (n2+ = 10–26) of the umbrella mode, ν2+, in the cation. From the energy separation between the two resolved peaks of each band, a value of 994 ± 16 cm−1 has been derived for the ν1+ breathing frequency of CF3+. For the high-lying n2+ levels, the apparent ν2+ umbrella spacing, 820 ± 14 cm−1, is fairly constant. Taking into account the ν2+ anharmonicity calculated by Bowman and coworkers, we have deduced ν2+ = 809 ± 14 cm−1, and semi-empirical estimations of the adiabatic ionization energy IEad.(CF3•) are proposed in good agreement with most of previous works. A value of the vertical ionization potential, IEvert.(CF3•) = 11.02 eV, has been derived from the observation of a photoelectron spectrum recorded at a fixed photon energy of 12 eV.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.60.+q Photoelectron spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Imaging ion-molecule reactions: Charge transfer and C-N bond formation in the C+ + NH3 system

Linsen Pei and James M. Farrar

J. Chem. Phys. 136, 204305 (2012); http://dx.doi.org/10.1063/1.4719808 (7 pages)

Online Publication Date: 23 May 2012

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The velocity mapping ion imaging method is applied to the ion-molecule reactions occurring between C+ and NH3. The velocity space images are collected over the relative collision energy range from 1.5 to 3.3 eV, allowing both product kinetic energy distributions and angular distributions to be obtained from the data. The charge transfer process appears to be direct, dominated by long-range electron transfer that results in minimal deflection of the products. The product kinetic energy distributions are consistent with a process dominated by energy resonance. The kinetic energy distributions for C-N bond formation appear to scale with the total available energy, providing strong evidence that energy in the [CNH3]+ precursor to products is distributed statistically. The angular distributions for C-N bond formation show pronounced forward-backward symmetry, as expected for a complex that resembles a prolate symmetric top decaying along its symmetry axis.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
33.15.Fm Bond strengths, dissociation energies
34.70.+e Charge transfer

Behavior of rate coefficients for ion-ion mutual neutralization, 300–550 K

Thomas M. Miller, Nicholas S. Shuman, and A. A. Viggiano

J. Chem. Phys. 136, 204306 (2012); http://dx.doi.org/10.1063/1.4720499 (7 pages)

Online Publication Date: 24 May 2012

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Rate coefficients kMN have been measured for a number of anion neutralization reactions with Ar+ and Kr+ over the temperature range 300–550 K. For the first time, the data set includes anions of radicals and other short-lived species. In the present paper, we review these results and make note of correlations with reduced mass, electron binding energy of the anion (equivalent to the electron affinity of the corresponding neutral), and temperature, and compare with expectations from absorbing sphere models. An intriguing result is that the data for diatomic anions neutralized by Ar+ and Kr+ have kMN values close to 3 × 10−8 cm3 s−1 at 300 K, a figure which is lower than those for all of the polyatomic anions at 300 K except for SF5− + Kr+. For the polyatomic anions studied here, neutralized by Ar+ and Kr+, the reduced mass dependence agrees with theory, on average, but we find a stronger temperature dependence of T−0.9 than expected from the theoretical E−0.5 energy dependence of the rate coefficient at thermal energies. The kMN show a weak dependence on the electron binding energy of the anion for the polyatomic species studied.
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82.20.Pm Rate constants, reaction cross sections, and activation energies
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Density and birefringence of a highly stable α,α,β-trisnaphthylbenzene glass

Shakeel S. Dalal, A. Sepúlveda, Greg K. Pribil, Zahra Fakhraai, and M. D. Ediger

J. Chem. Phys. 136, 204501 (2012); http://dx.doi.org/10.1063/1.4719532 (10 pages)

Online Publication Date: 23 May 2012

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Spectroscopic ellipsometry has been used to understand the properties of α,α,β-trisnaphthylbenzene (ααβ-TNB) glasses vapor-deposited at a substrate temperature of 295 K (0.85 Tg). In a single temperature ramping experiment, a range of properties of the as-deposited glass can be measured, including density, fictive temperature, onset temperature, thermal expansion coefficient, and birefringence. The vapor-deposited ααβ-TNB glass is 1.3% more dense than the ordinary glass prepared by cooling at 1 K/min, is found to be birefringent, has a fictive temperature 35 K below that of the ordinary glass, and an onset temperature 20 K above that of the ordinary glass. The thermal expansion coefficient of the vapor-deposited ααβ-TNB glass is 14% lower than that of the ordinary glass, indicating that lower portions of the potential energy landscape have more harmonic potential minima than the parts accessible to the ordinary glass.
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78.20.Fm Birefringence
78.66.Qn Polymers; organic compounds
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
68.55.am Polymers and organics
65.40.De Thermal expansion; thermomechanical effects

Dynamic relaxation of a liquid cavity under amorphous boundary conditions

Andrea Cavagna, Tomás S. Grigera, and Paolo Verrocchio

J. Chem. Phys. 136, 204502 (2012); http://dx.doi.org/10.1063/1.4720477 (16 pages)

Online Publication Date: 24 May 2012

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The growth of cooperatively rearranging regions was invoked long ago by Adam and Gibbs to explain the slowing down of glass-forming liquids. The lack of knowledge about the nature of the growing order, though, complicates the definition of an appropriate correlation function. One option is the point-to-set (PTS) correlation function, which measures the spatial span of the influence of amorphous boundary conditions on a confined system. By using a swap Monte Carlo algorithm we measure the equilibration time of a liquid droplet bounded by amorphous boundary conditions in a model glass-former at low temperature, and we show that the cavity relaxation time increases with the size of the droplet, saturating to the bulk value when the droplet outgrows the point-to-set correlation length. This fact supports the idea that the point-to-set correlation length is the natural size of the cooperatively rearranging regions. On the other hand, the cavity relaxation time computed by a standard, nonswap dynamics, has the opposite behavior, showing a very steep increase when the cavity size is decreased. We try to reconcile this difference by discussing the possible hybridization between mode-coupling theory and activated processes, and by introducing a new kind of amorphous boundary conditions, inspired by the concept of frozen external state as an alternative to the commonly used frozen external configuration.
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47.55.D- Drops and bubbles
61.20.Ja Computer simulation of liquid structure
64.60.My Metastable phases

Ultrafast photo-induced charge transfer unveiled by two-dimensional electronic spectroscopy

Oliver Bixner, Vladimír Lukeš, Tomáš Mančal, Jürgen Hauer, Franz Milota, Michael Fischer, Igor Pugliesi, Maximilian Bradler, Walther Schmid, Eberhard Riedle, Harald F. Kauffmann, and Niklas Christensson

J. Chem. Phys. 136, 204503 (2012); http://dx.doi.org/10.1063/1.4720492 (12 pages)

Online Publication Date: 24 May 2012

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The interaction of exciton and charge transfer (CT) states plays a central role in photo-induced CT processes in chemistry, biology, and physics. In this work, we use a combination of two-dimensional electronic spectroscopy (2D-ES), pump-probe measurements, and quantum chemistry to investigate the ultrafast CT dynamics in a lutetium bisphthalocyanine dimer in different oxidation states. It is found that in the anionic form, the combination of strong CT-exciton interaction and electronic asymmetry induced by a counter-ion enables CT between the two macrocycles of the complex on a 30 fs timescale. Following optical excitation, a chain of electron and hole transfer steps gives rise to characteristic cross-peak dynamics in the electronic 2D spectra, and we monitor how the excited state charge density ultimately localizes on the macrocycle closest to the counter-ion within 100 fs. A comparison with the dynamics in the radical species further elucidates how CT states modulate the electronic structure and tune fs-reaction dynamics. Our experiments demonstrate the unique capability of 2D-ES in combination with other methods to decipher ultrafast CT dynamics.
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34.70.+e Charge transfer
34.80.-i Electron and positron scattering
33.60.+q Photoelectron spectra
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.80.Be Level crossing and optical pumping

The electronic structure of free aluminum clusters: Metallicity and plasmons

Tomas Andersson, Chaofan Zhang, Maxim Tchaplyguine, Svante Svensson, Nils Mårtensson, and Olle Björneholm

J. Chem. Phys. 136, 204504 (2012); http://dx.doi.org/10.1063/1.4718362 (5 pages)

Online Publication Date: 24 May 2012

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The electronic structure of free aluminum clusters with ∼3–4 nm radius has been investigated using synchrotron radiation-based photoelectron and Auger electron spectroscopy. A beam of free clusters has been produced using a gas-aggregation source. The 2p core level and the valence band have been probed. Photoelectron energy-loss features corresponding to both bulk and surface plasmon excitation following photoionization of the 2p level have been observed, and the excitation energies have been derived. In contrast to some expectations, the loss features have been detected at energies very close to those of the macroscopic solid. The results are discussed from the point of view of metallic properties in nanoparticles with a finite number of constituent atoms.
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36.40.Cg Electronic and magnetic properties of clusters
32.80.Hd Auger effect (including Coster-Krönig transitions)
32.80.Fb Photoionization of atoms and ions
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Rotational dynamics in supercooled water from nuclear spin relaxation and molecular simulations

Johan Qvist, Carlos Mattea, Erik P. Sunde, and Bertil Halle

J. Chem. Phys. 136, 204505 (2012); http://dx.doi.org/10.1063/1.4720941 (16 pages)

Online Publication Date: 24 May 2012

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Structural dynamics in liquid water slow down dramatically in the supercooled regime. To shed further light on the origin of this super-Arrhenius temperature dependence, we report high-precision 17O and 2H NMR relaxation data for H2O and D2O, respectively, down to 37 K below the equilibrium freezing point. With the aid of molecular dynamics (MD) simulations, we provide a detailed analysis of the rotational motions probed by the NMR experiments. The NMR-derived rotational correlation time τR is the integral of a time correlation function (TCF) that, after a subpicosecond librational decay, can be described as a sum of two exponentials. Using a coarse-graining algorithm to map the MD trajectory on a continuous-time random walk (CTRW) in angular space, we show that the slowest TCF component can be attributed to large-angle molecular jumps. The mean jump angle is ∼48° at all temperatures and the waiting time distribution is non-exponential, implying dynamical heterogeneity. We have previously used an analogous CTRW model to analyze quasielastic neutron scattering data from supercooled water. Although the translational and rotational waiting times are of similar magnitude, most translational jumps are not synchronized with a rotational jump of the same molecule. The rotational waiting time has a stronger temperature dependence than the translation one, consistent with the strong increase of the experimentally derived product τR DT at low temperatures. The present CTRW jump model is related to, but differs in essential ways from the extended jump model proposed by Laage and co-workers. Our analysis traces the super-Arrhenius temperature dependence of τR to the rotational waiting time. We present arguments against interpreting this temperature dependence in terms of mode-coupling theory or in terms of mixture models of water structure.
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61.25.Em Molecular liquids
76.60.Es Relaxation effects
63.20.dk First-principles theory
61.20.Ja Computer simulation of liquid structure

Elasticity of two-dimensional crystals of polydisperse hard disks near close packing: Surprising behavior of the Poisson's ratio

Konstantin V. Tretiakov and Krzysztof W. Wojciechowski

J. Chem. Phys. 136, 204506 (2012); http://dx.doi.org/10.1063/1.4722100 (8 pages)

Online Publication Date: 24 May 2012

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The equation of state, elastic constants, and Poisson's ratio of a crystalline two-dimensional polydisperse hard disk system were determined in the close packing limit. Monte Carlo simulations in the NpT ensemble with variable shape of the periodic box reveal that the pressure and elastic constants grow with increasing polydispersity. The equation of state and the bulk modulus are well described by the free volume approximation. The latter approximation fails, however, for the shear modulus. The simulations also show that the introduction of any amount of size polydispersity in the hard disk systems causes a discontinuous “jump” of the Poisson's ratio in the close packing limit from the value νδ=0 = 0.1308(22), obtained for equidiameter hard disks, to νδ>0 ≈ 1, estimated for the polydisperse disks.
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46.25.Cc Theoretical studies
back to top Surfaces, Interfaces, and Materials

Fluorescence quenching near small metal nanoparticles

V. N. Pustovit and T. V. Shahbazyan

J. Chem. Phys. 136, 204701 (2012); http://dx.doi.org/10.1063/1.4721388 (6 pages)

Online Publication Date: 22 May 2012

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We develop a microscopic model for fluorescence of a molecule (or semiconductor quantum dot) near a small metal nanoparticle. When a molecule is situated close to metal surface, its fluorescence is quenched due to energy transfer to the metal. We perform quantum-mechanical calculations of energy transfer rates for nanometer-sized Au nanoparticles and find that nonlocal and quantum-size effects significantly enhance dissipation in metal as compared to those predicted by semiclassical electromagnetic models. However, the dependence of transfer rates on molecule's distance to metal nanoparticle surface, d, is significantly weaker than the d−4 behavior for flat metal surface with a sharp boundary predicted by previous calculations within random phase approximation.
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33.50.Dq Fluorescence and phosphorescence spectra
33.50.Hv Radiationless transitions, quenching
34.35.+a Interactions of atoms and molecules with surfaces
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
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