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21 Oct 2011

Volume 135, Issue 15, Articles (15xxxx)

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J. Chem. Phys. 135, 151101 (2011); http://dx.doi.org/10.1063/1.3653938 (4 pages)

Yang Jiao and Salvatore Torquato
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Communication: A packing of truncated tetrahedra that nearly fills all of space and its melting properties

Yang Jiao and Salvatore Torquato

J. Chem. Phys. 135, 151101 (2011); http://dx.doi.org/10.1063/1.3653938 (4 pages) | Cited 1 time

Online Publication Date: 17 October 2011

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Dense polyhedron packings are useful models of a variety of condensed matter and biological systems and have intrigued scientists and mathematicians for centuries. Here, we analytically construct the densest known packing of truncated tetrahedra with a remarkably high packing fraction ϕ = 207/208 = 0.995192…, which is amazingly close to unity and strongly implies its optimality. This construction is based on a generalized organizing principle for polyhedra lacking central symmetry that we introduce here. The “holes” in the putative optimal packing are perfect tetrahedra, which leads to a new tessellation of space by truncated tetrahedra and tetrahedra. Its packing characteristics and equilibrium melting properties as the system undergoes decompression are discussed.
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64.70.dj Melting of specific substances
82.70.Dd Colloids
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Communication: Quasiclassical trajectory calculations of correlated product-state distributions for the dissociation of (H2O)2 and (D2O)2

Gábor Czakó, Yimin Wang, and Joel M. Bowman

J. Chem. Phys. 135, 151102 (2011); http://dx.doi.org/10.1063/1.3655564 (4 pages)

Online Publication Date: 18 October 2011

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Stimulated by recent experiments [B. E. Rocher-Casterline, L. C. Ch'ng, A. K. Mollner, and H. Reisler, J. Chem. Phys. 134, 211101 (2011)], we report quasiclassical trajectory calculations of the dissociation dynamics of the water dimer, (H2O)2 (and also (D2O)2) using a full-dimensional ab initio potential energy surface. The dissociation is initiated by exciting the H-bonded OH(OD)-stretch, as done experimentally for (H2O)2. Normal mode analysis of the fragment pairs is done and the correlated vibrational populations are obtained by (a) standard histogram binning (HB), (b) harmonic normal-mode energy-based Gaussian binning (GB), and (c) a modified version of (b) using accurate vibrational energies obtained in the Cartesian space. We show that HB allows opening quantum mechanically closed states, whereas GB, especially via (c), gives physically correct results. Dissociation of both (H2O)2 and (D2O)2 mainly produces either fragment in the bending excited (010) state. The H2O(J) and D2O(J) rotational distributions are similar, peaking at J = 3–5. The computations do not show significant difference between the ro-vibrational distributions of the donor and acceptor fragments. Diffusion Monte Carlo computations are performed for (D2O)2 providing an accurate zero-point energy of 7247 cm−1, and thus, a benchmark D0 of 1244 ± 5 cm−1.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.30.Rs Hydrogen bonding, hydrophilic effects
82.20.Ej Quantum theory of reaction cross section
82.20.Fd Collision theories; trajectory models
82.20.Hf Product distribution
82.20.Kh Potential energy surfaces for chemical reactions
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Communication: Orbital instabilities and triplet states from time-dependent density functional theory and long-range corrected functionals

John S. Sears, Thomas Koerzdoerfer, Cai-Rong Zhang, and Jean-Luc Brédas

J. Chem. Phys. 135, 151103 (2011); http://dx.doi.org/10.1063/1.3656734 (4 pages) | Cited 1 time

Online Publication Date: 21 October 2011

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Long-range corrected hybrids represent an increasingly popular class of functionals for density functional theory (DFT) that have proven to be very successful for a wide range of chemical applications. In this Communication, we examine the performance of these functionals for time-dependent (TD)DFT descriptions of triplet excited states. Our results reveal that the triplet energies are particularly sensitive to the range-separation parameter; this sensitivity can be traced back to triplet instabilities in the ground state coming from the large effective amounts of Hartree-Fock exchange included in these functionals. As such, the use of standard long-range corrected functionals for the description of triplet states at the TDDFT level is not recommended.
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31.15.ee Time-dependent density functional theory
31.15.eg Exchange-correlation functionals (in current density functional theory)
31.15.xr Self-consistent-field methods
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back to top Theoretical Methods and Algorithms

Electric field polarization in conventional density functional theory: From quasilinear to two-dimensional and three-dimensional extended systems

Bernard Kirtman, Valentina Lacivita, Roberto Dovesi, and Heribert Reis

J. Chem. Phys. 135, 154101 (2011); http://dx.doi.org/10.1063/1.3649945 (10 pages) | Cited 1 time

Online Publication Date: 17 October 2011

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The large overshoot in (hyper)polarizabilities of quasilinear (1D) chains calculated by applying density functional theory with conventional functionals is investigated for several 2D and 3D extended systems. These systems include arrays of molecular hydrogen chains, as well as 2D coronene-type structures and LiF in 1D, 2D, and 3D. Contrary to a recently proposed model it is found that the overshoot persists in all of these cases. A simple explanation is provided by an analysis of the field-induced charges for molecular hydrogen, which shows an excessive buildup at the chain ends regardless of where the chain is located within the 2D and 3D array.
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31.15.E- Density-functional theory
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Bh General molecular conformation and symmetry; stereochemistry
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Entanglement of polar symmetric top molecules as candidate qubits

Qi Wei, Sabre Kais, Bretislav Friedrich, and Dudley Herschbach

J. Chem. Phys. 135, 154102 (2011); http://dx.doi.org/10.1063/1.3649949 (10 pages)

Online Publication Date: 17 October 2011

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Proposals for quantum computing using rotational states of polar molecules as qubits have previously considered only diatomic molecules. For these the Stark effect is second-order, so a sizable external electric field is required to produce the requisite dipole moments in the laboratory frame. Here we consider use of polar symmetric top molecules. These offer advantages resulting from a first-order Stark effect, which renders the effective dipole moments nearly independent of the field strength. That permits use of much lower external field strengths for addressing sites. Moreover, for a particular choice of qubits, the electric dipole interactions become isomorphous with NMR systems for which many techniques enhancing logic gate operations have been developed. Also inviting is the wider chemical scope, since many symmetric top organic molecules provide options for auxiliary storage qubits in spin and hyperfine structure or in internal rotation states.
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03.65.Ud Entanglement and quantum nonlocality (e.g. EPR paradox, Bell's inequalities, GHZ states, etc.)
03.67.Mn Entanglement measures, witnesses, and other characterizations
03.67.Lx Quantum computation architectures and implementations

Precise simulation of the freezing transition of supercritical Lennard-Jones

Michael Nayhouse, Ankur M. Amlani, and G. Orkoulas

J. Chem. Phys. 135, 154103 (2011); http://dx.doi.org/10.1063/1.3651193 (10 pages) | Cited 1 time

Online Publication Date: 17 October 2011

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The fluid-solid transition of the Lennard-Jones model is analyzed along a supercritical isotherm. The analysis is implemented via a simulation method which is based on a modification of the constrained cell model of Hoover and Ree. In the context of hard-sphere freezing, Hoover and Ree simulated the solid phase using a constrained cell model in which each particle is confined within its own Wigner-Seitz cell. Hoover and Ree also proposed a modified cell model by considering the effect of an external field of variable strength. High-field values favor configurations with a single particle per Wigner-Seitz cell and thus stabilize the solid phase. In previous work, a simulation method for freezing transitions, based on constant-pressure simulations of the modified cell model, was developed and tested on a system of hard spheres. In the present work, this method is used to determine the freezing transition of a Lennard-Jones model system on a supercritical isotherm at a reduced temperature of 2. As in the case of hard spheres, constant-pressure simulations of the fully occupied constrained cell model of a system of Lennard-Jones particles indicate a point of mechanical instability at a density which is approximately 70% of the density at close packing. Furthermore, constant-pressure simulations of the modified cell model indicate that as the strength of the field is reduced, the transition from the solid to the fluid is continuous below the mechanical instability point and discontinuous above. The fluid-solid transition of the Lennard-Jones system is obtained by analyzing the field-induced fluid-solid transition of the modified cell model in the high-pressure, zero-field limit. The simulations are implemented under constant pressure using tempering and histogram reweighting techniques. The coexistence pressure and densities are determined through finite-size scaling techniques for first-order phase transitions which are based on analyzing the size-dependent behavior of susceptibilities and dimensionless moment ratios of the order parameter.
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64.70.D- Solid-liquid transitions

All-electron time-dependent density functional theory with finite elements: Time-propagation approach

Lauri Lehtovaara, Ville Havu, and Martti Puska

J. Chem. Phys. 135, 154104 (2011); http://dx.doi.org/10.1063/1.3651239 (8 pages)

Online Publication Date: 17 October 2011

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We present an all-electron method for time-dependent density functional theory which employs hierarchical nonuniform finite-element bases and the time-propagation approach. The method is capable of treating linear and nonlinear response of valence and core electrons to an external field. We also introduce (i) a preconditioner for the propagation equation, (ii) a stable way to implement absorbing boundary conditions, and (iii) a new kind of absorbing boundary condition inspired by perfectly matched layers.
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31.15.ee Time-dependent density functional theory
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Calculating dispersion interactions using maximally localized Wannier functions

Lampros Andrinopoulos, Nicholas D. M. Hine, and Arash A. Mostofi

J. Chem. Phys. 135, 154105 (2011); http://dx.doi.org/10.1063/1.3647912 (13 pages)

Online Publication Date: 17 October 2011

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We investigate a recently developed approach [P. L. Silvestrelli, Phys. Rev. Lett. 100, 053002 (2008)10.1103/PhysRevLett.100.053002; J. Phys. Chem. A 113, 5224 (2009)] that uses maximally localized Wannier functions to evaluate the van der Waals contribution to the total energy of a system calculated with density-functional theory. We test it on a set of atomic and molecular dimers of increasing complexity (argon, methane, ethene, benzene, phthalocyanine, and copper phthalocyanine) and demonstrate that the method, as originally proposed, has a number of shortcomings that hamper its predictive power. In order to overcome these problems, we have developed and implemented a number of improvements to the method and show that these modifications give rise to calculated binding energies and equilibrium geometries that are in closer agreement to results of quantum-chemical coupled-cluster calculations.
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34.20.Gj Intermolecular and atom-molecule potentials and forces
34.20.Cf Interatomic potentials and forces
31.15.E- Density-functional theory
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Dj Interatomic distances and angles

A fast variational Gaussian wavepacket method: Size-induced structural transitions in large neon clusters

Ionuţ Georgescu and Vladimir A. Mandelshtam

J. Chem. Phys. 135, 154106 (2011); http://dx.doi.org/10.1063/1.3651473 (6 pages)

Online Publication Date: 19 October 2011

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The variational Gaussian wavepacket (VGW) approximation provides an alternative to path integral Monte Carlo for the computation of thermodynamic properties of many-body systems at thermal equilibrium. It provides a direct access to the thermal density matrix and is particularly efficient for Monte Carlo approaches, as for an N-body system it operates in a non-inflated 3N-dimensional configuration space. Here, we greatly accelerate the VGW method by retaining only the relevant short-range correlations in the (otherwise full) 3N × 3N Gaussian width matrix without sacrificing the accuracy of the fully coupled VGW method. This results in the reduction of the original O(N3) scaling to O(N2). The fast-VGW method is then applied to quantum Lennard-Jones clusters with sizes up to N = 6500 atoms. Following Doye and Calvo [JCP 116, 8307 (2002)10.1063/1.1469616] we study the competition between the icosahedral and decahedral structural motifs in NeN clusters as a function of N.
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36.40.Mr Spectroscopy and geometrical structure of clusters
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
31.15.V- Electron correlation calculations for atoms, ions and molecules

Vibrational contributions to cubic response functions from vibrational configuration interaction response theory

Mikkel Bo Hansen and Ove Christiansen

J. Chem. Phys. 135, 154107 (2011); http://dx.doi.org/10.1063/1.3652895 (14 pages)

Online Publication Date: 20 October 2011

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In continuation of our recent paper on vibrational quadratic response functions for vibrational configuration interaction wave functions, we present in this paper a derivation and implementation of the pure vibrational cubic response function for vibrational configuration interaction wave functions. In addition, we present combined electronic and vibrational cubic response functions derived from sum-over-states expressions in the Born-Oppenheimer framework and a discussion of complicating issues. The implementation enables analytic calculation of the pure vibrational cubic response function via response theory, which constitutes a part of the vibronic cubic response function.
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33.20.Tp Vibrational analysis
31.15.V- Electron correlation calculations for atoms, ions and molecules

Rapid, accurate calculation of the s-wave scattering length

Vladimir V. Meshkov, Andrey V. Stolyarov, and Robert J. Le Roy

J. Chem. Phys. 135, 154108 (2011); http://dx.doi.org/10.1063/1.3649946 (11 pages)

Online Publication Date: 20 October 2011

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Transformation of the conventional radial Schrödinger equation defined on the interval  r ∈ [0, ∞) into an equivalent form defined on the finite domain  y(r) ∈ [a, b]  allows the s-wave scattering length as to be exactly expressed in terms of a logarithmic derivative of the transformed wave function ϕ(y) at the outer boundary point y = b, which corresponds to r = ∞. In particular, for an arbitrary interaction potential that dies off as fast as 1/rn for n ⩾ 4, the modified wave function ϕ(y) obtained by using the two-parameter mapping function r(y;math,β) = math[1+mathtan(πy/2)] has no singularities, and as = math[1+mathmathmath]. For a well bound potential with equilibrium distance re, the optimal mapping parameters are  math ≈ re  and  β ≈ math−1. An outward integration procedure based on Johnson's log-derivative algorithm [J. Comp. Phys. 13, 445 (1973)] combined with a Richardson extrapolation procedure is shown to readily yield high precision as-values both for model Lennard-Jones (2n, n) potentials and for realistic published potentials for the Xe–e−, Cs 2(a3Σu+), and 3, 4 He 2(X1Σg+) systems. Use of this same transformed Schrödinger equation was previously shown [V. V. Meshkov et al., Phys. Rev. A 78, 052510 (2008)] to ensure the efficient calculation of all bound levels supported by a potential, including those lying extremely close to dissociation.
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03.65.Ge Solutions of wave equations: bound states

Controlling the transmission line shape of molecular t-stubs and potential thermoelectric applications

Robert Stadler and Troels Markussen

J. Chem. Phys. 135, 154109 (2011); http://dx.doi.org/10.1063/1.3653790 (10 pages)

Online Publication Date: 20 October 2011

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Asymmetric line shapes can occur in the transmission function describing electron transport in the vicinity of a minimum caused by quantum interference effects. Such asymmetry can be used to increase the thermoelectric efficiency of molecular junctions. So far, however, asymmetric line shapes have been only empirically found for just a few rather complex organic molecules where the origins of the line shapes relation to molecular structure were not resolved. In the present, work we introduce a method to analyze the structure dependence of the asymmetry of interference dips from simple two site tight-binding models, where one site corresponds to a molecular π orbital of the wire and the other to an atomic pz orbital of a side group, which allows us to characterize analytically the peak shape in terms of just two parameters. We assess our scheme with first-principles electron transport calculations for a variety of t-stub molecules and also address their suitability for thermoelectric applications.
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85.65.+h Molecular electronic devices
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)

Reaction coordinates for the crystal nucleation of colloidal suspensions extracted from the reweighted path ensemble

Wolfgang Lechner, Christoph Dellago, and Peter G. Bolhuis

J. Chem. Phys. 135, 154110 (2011); http://dx.doi.org/10.1063/1.3651367 (14 pages)

Online Publication Date: 20 October 2011

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We study the mechanisms of the homogeneous crystal nucleation from the supercooled liquid to the crystal phase in the Gaussian core model for colloidal suspensions with the aim to find optimal reaction coordinates. We introduce a set of novel collective variables based on the local structure of particles. By applying likelihood maximization of the committor function for the reweighted path ensemble constructed by replica exchange transition interface sampling, we select the optimal reaction coordinates from the set of collective variables. We find that the size of the cloud of prestructured particles surrounding the crystalline nucleus enhances the description of the transition. Further, we show that the rearrangement of the inner core of the nucleus according to Ostwald's step rule is a separate process, independent of the growth of the nucleus.
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64.60.qe General theory and computer simulations of nucleation
82.70.Dd Colloids
82.70.Kj Emulsions and suspensions

Evaluation of coupling terms between intra- and intermolecular vibrations in coarse-grained normal-mode analysis: Does a stronger acid make a stiffer hydrogen bond?

Hirohiko Houjou

J. Chem. Phys. 135, 154111 (2011); http://dx.doi.org/10.1063/1.3652102 (9 pages)

Online Publication Date: 20 October 2011

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Using theory of harmonic normal-mode vibration analysis, we developed a procedure for evaluating the anisotropic stiffness of intermolecular forces. Our scheme for coarse-graining of molecular motions is modified so as to account for intramolecular vibrations in addition to relative translational/rotational displacement. We applied this new analytical scheme to four carboxylic acid dimers, for which coupling between intra- and intermolecular vibrations is crucial for determining the apparent stiffness of the intermolecular double hydrogen bond. The apparent stiffness constant was analyzed on the basis of a conjunct spring model, which defines contributions from true intermolecular stiffness and molecular internal stiffness. Consequently, the true intermolecular stiffness was in the range of 43–48 N m−1 for all carboxylic acids studied, regardless of the molecules’ acidity. We concluded that the difference in the apparent stiffness can be attributed to differences in the internal stiffness of the respective molecules.
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34.20.Gj Intermolecular and atom-molecule potentials and forces
33.15.Fm Bond strengths, dissociation energies
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.xr Self-consistent-field methods

Electronic excitation dynamics in multichromophoric systems described via a polaron-representation master equation

Avinash Kolli, Ahsan Nazir, and Alexandra Olaya-Castro

J. Chem. Phys. 135, 154112 (2011); http://dx.doi.org/10.1063/1.3652227 (13 pages) | Cited 1 time

Online Publication Date: 20 October 2011

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We derive a many-site version of the non-Markovian time-convolutionless polaron master equation [Jang et al., J. Chem Phys. 129, 101104 (2008)]10.1063/1.2977974 to describe electronic excitation dynamics in multichromophoric systems. By treating electronic and vibrational degrees of freedom in a combined frame (polaron frame), this theory is capable of interpolating between weak and strong exciton-phonon coupling and is able to account for initial non-equilibrium bath states and spatially correlated environments. Besides outlining a general expression for the expected value of any electronic system observable in the original frame, we also discuss implications of the Markovian and Secular approximations highlighting that they need not hold in the untransformed frame despite being strictly satisfied in the polaron frame. The key features of the theory are illustrated using as an example a four-site subsystem of the Fenna-Mathews-Olson light-harvesting complex. For a spectral density including a localised mode, we show that oscillations of site populations may only be observed when non-equilibrium bath effects are taken into account. Furthermore, we illustrate how this formalism allows us to identify the electronic and vibrational components of the oscillatory dynamics.
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71.38.-k Polarons and electron-phonon interactions
71.35.-y Excitons and related phenomena
63.20.Pw Localized modes
63.20.kk Phonon interactions with other quasiparticles

Intermediate state representation approach to physical properties of dicationic states

Yasen Velkov, Tsveta Miteva, Nicolas Sisourat, and Jochen Schirmer

J. Chem. Phys. 135, 154113 (2011); http://dx.doi.org/10.1063/1.3653229 (10 pages)

Online Publication Date: 21 October 2011

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The second-order algebraic construction (ADC(2)) approach to the two-particle (pp) propagator, devised to compute double ionization energies and associated spectroscopic amplitudes, is reformulated and extended using the concept of intermediate state representations (ISR). The ISR formulation allows one to go beyond the general limitations inherent to the propagator approach, as here (N−2)-electron wave functions and properties become directly accessible. The (N−2)-electron ISR(2) equations for a general one-particle operator have been derived and implemented in a recent version of the double ionization ADC(2) program. As a first test of the method, the dipole moments of a series of 2h states of LiH, HF, and H2O were computed and compared to the results of a full configuration interaction (FCI) treatment. The dipole moments obtained at the ADC(2)/ISR(2) computational level are in good agreement with the FCI results.
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34.50.Gb Electronic excitation and ionization of molecules
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
31.15.vn Electron correlation calculations for diatomic molecules

Time-averaging approximation in the interaction picture: Absorption line shapes for coupled chromophores with application to liquid water

Mino Yang and J. L. Skinner

J. Chem. Phys. 135, 154114 (2011); http://dx.doi.org/10.1063/1.3654005 (6 pages)

Online Publication Date: 21 October 2011

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The time-averaging approximation (TAA), originally developed to calculate vibrational line shapes for coupled chromophores using mixed quantum/classical methods, is reformulated. In the original version of the theory, time averaging was performed for the full one-exciton Hamiltonian, while herein the time averaging is performed on the coupling (off-diagonal) Hamiltonian in the interaction picture. As a result, the influence of the dynamic fluctuations of the transition energies is more accurately described. We compare numerical results of the two versions of the TAA with numerically exact results for the vibrational absorption line shape of the OH stretching modes in neat water. It is shown that the TAA in the interaction picture yields theoretical line shapes that are in better agreement with exact results.
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78.30.C- Liquids
71.35.-y Excitons and related phenomena
63.50.-x Vibrational states in disordered systems

Physical interpretation of mean local accumulation time of morphogen gradient formation

Alexander M. Berezhkovskii and Stanislav Y. Shvartsman

J. Chem. Phys. 135, 154115 (2011); http://dx.doi.org/10.1063/1.3654159 (6 pages)

Online Publication Date: 21 October 2011

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The paper deals with a reaction-diffusion problem that arises in developmental biology when describing the formation of the concentration profiles of signaling molecules, called morphogens, which control gene expression and, hence, cell differentiation. The mean local accumulation time, which is the mean time required to reach the steady state at a fixed point of a patterned tissue, is an important characteristic of the formation process. We show that this time is a sum of two times, the conditional mean first-passage time from the source to the observation point and the mean local accumulation time in the situation when the source is localized at the observation point.
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87.15.Vv Diffusion
back to top Advanced Experimental Techniques

Sizes of large He droplets

Luis F. Gomez, Evgeny Loginov, Russell Sliter, and Andrey F. Vilesov

J. Chem. Phys. 135, 154201 (2011); http://dx.doi.org/10.1063/1.3650235 (9 pages)

Online Publication Date: 18 October 2011

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Helium droplets spanning a wide size range, NHe = 103–1010, were formed in a continuous-nozzle beam expansion at different nozzle temperatures and a constant stagnation pressure of 20 bars. The average sizes of the droplets have been obtained by attenuation of the droplet beam through collisions with argon and helium gases at room temperature. The results obtained are in good agreement with previous measurements in the size range NHe = 105–107. Moreover, the measurements give the average sizes in the previously uncharacterized range of very large droplets of 107–1010 atoms. The droplet sizes and beam flux increase rapidly at nozzle temperatures below 6 K, which is ascribed to the formation of droplets within the nozzle interior. The mass spectra of the droplet beam upon electron impact ionization have also been obtained. The spectra show a large increase in the intensity of the He4+ signal upon increase of the droplet size, an effect which can be used as a secondary size standard in the droplet size range NHe = 104–109 atoms.
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34.80.Dp Atomic excitation and ionization
32.10.Bi Atomic masses, mass spectra, abundances, and isotopes
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Simulations of light induced processes in water based on ab initio path integrals molecular dynamics. I. Photoabsorption

Ondřej Svoboda, Milan Ončák, and Petr Slavíček

J. Chem. Phys. 135, 154301 (2011); http://dx.doi.org/10.1063/1.3649942 (16 pages)

Online Publication Date: 17 October 2011

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We have performed large-scale simulations of UV absorption spectra of water clusters (monomer to octamer) using a combination of ab initio path-integral molecular dynamics with reflection principle. The aim of the present work is four-fold: (1) To explore the transition from isolated molecules to bulk water from the perspective of UV photoabsorption. (2) To investigate quantum nuclear and thermal effects on the shape of the water UV spectra. (3) To make an assessment of the density functional theory functionals to be used for water excited states. (4) To check the applicability of the QM/MM schemes for a description of the UV absorption. Within the path integral molecular dynamics (PIMD)/reflection principle approach both the thermal and quantum vibrational effects including anharmonicities are accounted for. We demonstrate that shape of the spectra is primarily controlled by the nuclear quantum effects. The excited states and transition characteristics of the water clusters were calculated with the time-dependent density functional theory and equation-of-motion coupled clusters singles and doubles methods. Based on our benchmark calculations considering the whole UV spectrum we argue that the BHandHLYP method performs best among the 6 functionals tested (B3LYP, BHandHLYP, BNL, CAM-B3LYP, LC-ωPBE, and M06HF). We observe a gradual blueshift of the maximum of the first absorption peak with the increasing cluster size. The UV absorption spectrum for the finite size clusters (i.e., the peak centers, peak widths, and photoabsorption cross section) essentially converges into the corresponding bulk water spectrum. The effect of distant molecules accounted for within the polarizable continuum model is shown to be almost negligible. Using the natural transition orbitals we demonstrate that the first absorption band is formed by localized excitations while the second band includes delocalized excited states. Consequently, the QM/MM electrostatic embedding scheme can only be used for the modeling of the low energy part of the spectrum.
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36.40.Cg Electronic and magnetic properties of clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
33.20.Tp Vibrational analysis
33.20.Lg Ultraviolet spectra
31.15.ee Time-dependent density functional theory
31.15.at Molecule transport characteristics; molecular dynamics; electronic structure of polymers

Simulations of light induced processes in water based on ab initio path integrals molecular dynamics. II. Photoionization

Ondřej Svoboda, Milan Ončák, and Petr Slavíček

J. Chem. Phys. 135, 154302 (2011); http://dx.doi.org/10.1063/1.3649943 (13 pages)

Online Publication Date: 17 October 2011

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We have applied ab initio based reflection principle to simulate photoelectron spectra of small water clusters, ranging from monomer to octamer. The role of quantum and thermal effects on the structure of the water photoelectron spectra is discussed within the ab initio path integral molecular dynamics (PIMD) framework. We have used the PIMD method with up to 40 beads to sample the ground state quantum distribution at temperature T = 180 K. We have thoroughly tested the performance of various density functionals (B3LYP, BHandHLYP, M06HF, BNL, LC-ωPBE, and CAM-B3LYP) for the ionization process description. The benchmarking based on a comparison of simulated photoelectron spectra to experimental data and high level equation-of-motion ionization potential coupled clusters with singles and doubles calculations has singled out the BHandHLYP and LC-ωPBE functionals as the most reliable ones for simulations of light induced processes in water. The good performance of the density functional theory functionals to model the water photoelectron spectra also reflects their ability to reliably describe open shell excited states. The width of the photoelectron spectrum converges quickly with the cluster size as it is controlled by specific interactions of local character. The peak position is, on the other hand, defined by long-range non-specific solvent effects; it therefore only slowly converges to the corresponding bulk value. We are able to reproduce the experimental valence photoelectron spectrum of liquid water within the combined model of the water octamer embedded in a polarizable dielectric continuum. We demonstrate that including the long-range polarization and the state-specific treatment of the solvent response are needed for a reliable liquid water ionization description.
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36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Cg Electronic and magnetic properties of clusters
33.80.Eh Autoionization, photoionization, and photodetachment
33.70.Jg Line and band widths, shapes, and shifts
33.60.+q Photoelectron spectra
31.15.ae Electronic structure and bonding characteristics

Accurate quantum mechanical study of the Renner-Teller effect in the singlet CH2

Zhijun Zhang, Haitao Ma, and Wensheng Bian

J. Chem. Phys. 135, 154303 (2011); http://dx.doi.org/10.1063/1.3651081 (10 pages)

Online Publication Date: 18 October 2011

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The Renner-Teller (RT) effect between the two low-lying electronic states of singlet CH2, math1A1 and math1B1, is studied using the multi-configuration time-dependent Hartree method with complete treatment of the RT terms. The RT terms, which are the matrix elements of the electronic orbital angular momentum operators, are calculated with ab initio methods and fitted to analytical functions. The ro-vibronic energy levels with complete treatment and constant approximation of the RT terms are calculated and compared. The influences of the geometry dependence of the RT terms on the ro-vibronic energy levels are discussed. The differences of the variation trends and influences of the RT terms between CH2 and NH2 are explored. In particular, as the molecule bends from linearity, the curve of the RT term (〈math1A1|mathz2|math1A1〉) first goes down to reach a minimum and then goes up leading to decreased zero point energy and bending energy levels for the lower state of CH2 in contrast to the case of NH2.
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31.15.xr Self-consistent-field methods
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions

Microwave measurements of proton tunneling and structural parameters for the propiolic acid–formic acid dimer

Adam M. Daly, Kevin O. Douglass, Laszlo C. Sarkozy, Justin L. Neill, Matt T. Muckle, Daniel P. Zaleski, Brooks H. Pate, and Stephen G. Kukolich

J. Chem. Phys. 135, 154304 (2011); http://dx.doi.org/10.1063/1.3643720 (12 pages)

Online Publication Date: 19 October 2011

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Microwave spectra of the propiolic acid–formic acid doubly hydrogen bonded complex were measured in the 1 GHz to 21 GHz range using four different Fourier transform spectrometers. Rotational spectra for seven isotopologues were obtained. For the parent isotopologue, a total of 138 a-dipole transitions and 28 b-dipole transitions were measured for which the a-dipole transitions exhibited splittings of a few MHz into pairs of lines and the b-type dipole transitions were split by ∼580 MHz. The transitions assigned to this complex were fit to obtain rotational and distortion constants for both tunneling levels: A0+ = 6005.289(8), B0+ = 930.553(8), C0+ = 803.9948(6) MHz, Δ0+J = 0.075(1), Δ0+JK = 0.71(1), and δ0+j = −0.010(1) kHz and A0− = 6005.275(8), B0− = 930.546(8), C0− = 803.9907(5) MHz, Δ0−J = 0.076(1), Δ0−JK = 0.70(2), and δ0−j = −0.008(1) kHz. Double resonance experiments were used on some transitions to verify assignments and to obtain splittings for cases when the b-dipole transitions were difficult to measure. The experimental difference in energy between the two tunneling states is 291.428(5) MHz for proton-proton exchange and 3.35(2) MHz for the deuterium-deuterium exchange. The vibration-rotation coupling constant between the two levels, Fab, is 120.7(2) MHz for the proton-proton exchange. With one deuterium atom substituted in either of the hydrogen-bonding protons, the tunneling splittings were not observed for a-dipole transitions, supporting the assignment of the splitting to the concerted proton tunneling motion. The spectra were obtained using three Flygare-Balle type spectrometers and one chirped-pulse machine at the University of Virginia. Rotational constants and centrifugal distortion constants were obtained for HCOOH···HOOCCCH, H13COOH···HOOCCCH, HCOOD···HOOCCCH, HCOOH···DOOCCCH, HCOOD···DOOCCCH, DCOOH···HOOCCCH, and DCOOD···HOOCCCH. High-level ab initio calculations provided initial rotational constants for the complex, structural parameters, and some details of the proton tunneling potential energy surface. A least squares fit to the isotopic data reveals a planar structure that is slightly asymmetric in the OH distances. The formic OH···O propiolic hydrogen bond length is 1.8 Å and the propiolic OH···O formic hydrogen bond length is 1.6 Å, for the equilibrium configuration. The magnitude of the dipole moment was experimentally determined to be 1.95(3) × 10−30 C m (0.584(8) D) for the 0+ states and 1.92(5) × 10−30 C m (0.576(14) D) for the 0− states.
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33.20.Bx Radio-frequency and microwave spectra
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
31.15.ae Electronic structure and bonding characteristics
31.30.jp Electron electric dipole moment
31.30.Gs Hyperfine interactions and isotope effects
33.15.Fm Bond strengths, dissociation energies

Decomposition of pentaerythritol tetranitrate [C(CH2ONO2)4] following electronic excitation

Zijun Yu and Elliot R. Bernstein

J. Chem. Phys. 135, 154305 (2011); http://dx.doi.org/10.1063/1.3652893 (10 pages)

Online Publication Date: 20 October 2011

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We report the experimental and theoretical study of the decomposition of gas phase pentaerythritol tetranitrate (PETN) [C(CH2ONO2)4] following electronic state excitation. PETN has received major attention as an insensitive, high energy explosive; however, the mechanism and dynamics of the decomposition of this material are not clear yet. The initial decomposition mechanism of PETN is explored with nanosecond energy resolved spectroscopy and quantum chemical theory employing the ONIOM algorithm at the complete active space self-consistent field (CASSCF) level. The nitric oxide (NO) molecule is observed as an initial decomposition product from PETN at three UV excitation wavelengths (226, 236, and 248 nm) with a pulse duration of 8 ns. Energies of the three excitation wavelengths coincide with the (0–0), (0–1), and (0–2) vibronic bands of the NO A 2Σ+ ← X 2Π electronic transition, respectively. A unique excitation wavelength independent dissociation channel is observed for PETN, which generates the NO product with a rotationally cold (∼20 K) and a vibrationally hot (∼1300 K) distribution. Potential energy surface calculations at the ONIOM(CASSCF:UFF) level of theory illustrate that conical intersections play an important role in the decomposition mechanism. Electronically excited S1 PETN returns to the ground state through the (S1/S0)CI conical intersection, and undergoes a nitro-nitrite isomerization to generate the NO product.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.30.Qt Isomerization and rearrangement
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Hf Product distribution
82.20.Fd Collision theories; trajectory models

A 4D wave packet study of the CH3I photodissociation in the A-band. Comparison with femtosecond velocity map imaging experiments

A. García-Vela, R. de Nalda, J. Durá, J. González-Vázquez, and L. Bañares

J. Chem. Phys. 135, 154306 (2011); http://dx.doi.org/10.1063/1.3650718 (12 pages)

Online Publication Date: 20 October 2011

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The time-resolved photodissociation dynamics of CH3I in the A-band has been studied theoretically using a wave packet model including four degrees of freedom, namely the C–I dissociation coordinate, the I–CH3 bending mode, the CH3 umbrella mode, and the C−H symmetric stretch mode. Clocking times and final product state distributions of the different dissociation (nonadiabatic) channels yielding spin-orbit ground and excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch ν1 and umbrella ν2 modes) CH3 fragments have been obtained and compared with the results of femtosecond velocity map imaging experiments. The wave packet calculations are able to reproduce with very good agreement the experimental reaction times for the CH3(ν1, ν2)+I*(2P1/2) dissociation channels with ν1 = 0 and ν2 = 0,1,2, and also for the channel CH3(ν1 = 0, ν2 = 0)+I(2P3/2). However, the model fails to predict the experimental clocking times for the CH3(ν1, ν2)+I(2P3/2) channels with (ν1, ν2) = (0, 1), (0, 2), and (1, 0), that is, when the CH3 fragment produced along with spin-orbit ground state I atoms is vibrationally excited. These results are similar to those previously obtained with a three-dimensional wave packet model, whose validity is discussed in the light of the results of the four-dimensional treatment. Possible explanations for the disagreements found between theory and experiment are also discussed.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Bh General molecular conformation and symmetry; stereochemistry
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