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28 Jul 2010

Volume 133, Issue 4, Articles (04xxxx)

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J. Chem. Phys. 133, 044901 (2010); http://dx.doi.org/10.1063/1.3436517 (11 pages)

Ian M. Craig, Christopher J. Tassone, Sarah H. Tolbert, and Benjamin J. Schwartz
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Communication: High-frequency acoustic excitations and boson peak in glasses: A study of their temperature dependence

B. Ruta, G. Baldi, V. M. Giordano, L. Orsingher, S. Rols, F. Scarponi, and G. Monaco

J. Chem. Phys. 133, 041101 (2010); http://dx.doi.org/10.1063/1.3460815 (4 pages) | Cited 4 times

Online Publication Date: 27 July 2010

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The results of a combined experimental study of the high-frequency acoustic dynamics and of the vibrational density of states (VDOS) as a function of temperature in a glass of sorbitol are reported here. The excess in the VDOS at ∼ 4.5 meV over the Debye, elastic continuum prediction (boson peak) is found to be clearly related to anomalies observed in the acoustic dispersion curve in the mesoscopic wavenumber range of few nm−1. The quasiharmonic temperature dependence of the acoustic dispersion curves offers a natural explanation for the observed scaling of the boson peak with the elastic medium properties.
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62.65.+k Acoustical properties of solids
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
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Communication: Electric properties of the ThO(X 1Σ+) molecule

Alexei A. Buchachenko

J. Chem. Phys. 133, 041102 (2010); http://dx.doi.org/10.1063/1.3459888 (3 pages) | Cited 2 times

Online Publication Date: 29 July 2010

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To assist the proposed search for the electric dipole moment of the electron with the thorium oxide, converged ab initio coupled cluster calculations are performed on the interaction energy, dipole moment, anisotropic static dipole polarizability, and quadrupole moment of the ThO(X 1Σ+) molecule as functions of internuclear distance. The rovibrational energy levels and wave functions are computed to derive the spectroscopic constants and matrix elements of electric properties. Ab initio calculations provide good agreement with the measured spectroscopic constants but call for a revision of the dissociation energy estimates from mass-spectrometric measurements and previous calculations.
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31.15.A- Ab initio calculations
31.15.bw Coupled-cluster theory
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Fm Bond strengths, dissociation energies
33.15.Ta Mass spectra
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back to top Theoretical Methods and Algorithms

Insights into the orbital invariance problem in state-specific multireference coupled cluster theory

Francesco A. Evangelista and Jürgen Gauss

J. Chem. Phys. 133, 044101 (2010); http://dx.doi.org/10.1063/1.3456546 (5 pages) | Cited 25 times

Online Publication Date: 22 July 2010

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In this communication we report the results of our studies on the orbital invariance properties of the state-specific multireference coupled cluster approach suggested by Mukherjee and co-workers (Mk-MRCC). In particular, we have gathered numerical evidence to show that even when the linear excitation manifold is modified in order to span the same space for each reference, the resulting method is not orbital invariant. In order to test this conjecture we have proposed a new truncation scheme (Mk-MRCCSDtq) which, in addition to full single and double excitations, contains partial triple and quadruple excitations. For a reference space generated by all possible combinations of two electrons in two orbitals, the linear excitation manifold of Mk-MRCCSDtq spans the same set for each reference determinant. Mk-MRCCSDtq is found to lack energy invariance for rotations among active molecular orbitals but it is less sensitive to orbital rotations than the conventional scheme which includes only singles and doubles (Mk-MRCCSD). Nevertheless, Mk-MRCCSDtq is a very accurate method, superior with respect to multireference configuration interaction approaches, and competitive with the active-space coupled cluster method and the MRexpT ansatz.
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31.15.bw Coupled-cluster theory

An efficient density-functional-theory force evaluation for large molecular systems

Simen Reine, Andreas Krapp, Maria Francesca Iozzi, Vebjørn Bakken, Trygve Helgaker, Filip Pawłowski, and Pawel Sałek

J. Chem. Phys. 133, 044102 (2010); http://dx.doi.org/10.1063/1.3459061 (9 pages) | Cited 2 times

Online Publication Date: 22 July 2010

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An efficient, linear-scaling implementation of Kohn–Sham density-functional theory for the calculation of molecular forces for systems containing hundreds of atoms is presented. The density-fitted Coulomb force contribution is calculated in linear time by combining atomic integral screening with the continuous fast multipole method. For higher efficiency and greater simplicity, the near-field Coulomb force contribution is calculated by expanding the solid-harmonic Gaussian basis functions in Hermite rather than Cartesian Gaussians. The efficiency and linear complexity of the molecular-force evaluation is demonstrated by sample calculations and applied to the geometry optimization of a few selected large systems.
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31.15.E- Density-functional theory
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.-p Calculations and mathematical techniques in atomic and molecular physics

Ab initio statistical mechanics of surface adsorption and desorption. II. Nuclear quantum effects

D. Alfè and M. J. Gillan

J. Chem. Phys. 133, 044103 (2010); http://dx.doi.org/10.1063/1.3466919 (9 pages) | Cited 1 time

Online Publication Date: 22 July 2010

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We show how the path-integral formulation of quantum statistical mechanics can be used to construct practical ab initio techniques for computing the chemical potential of molecules adsorbed on surfaces, with full inclusion of quantum nuclear effects. The techniques we describe are based on the computation of the potential of mean force on a chosen molecule and generalize the techniques developed recently for classical nuclei. We present practical calculations based on density functional theory with a generalized-gradient exchange-correlation functional for the case of H2O on the MgO (001) surface at low coverage. We note that the very high vibrational frequencies of the H2O molecule would normally require very large numbers of time slices (beads) in path-integral calculations, but we show that this requirement can be dramatically reduced by employing the idea of thermodynamic integration with respect to the number of beads. The validity and correctness of our path-integral calculations on the H2O/MgO(001) system are demonstrated by supporting calculations on a set of simple model systems for which quantum contributions to the free energy are known exactly from analytic arguments.
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68.43.Mn Adsorption kinetics
68.43.Nr Desorption kinetics
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.
05.30.-d Quantum statistical mechanics
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Accurate calculations of the hydration free energies of druglike molecules using the reference interaction site model

David S. Palmer, Volodymyr P. Sergiievskyi, Frank Jensen, and Maxim V. Fedorov

J. Chem. Phys. 133, 044104 (2010); http://dx.doi.org/10.1063/1.3458798 (11 pages) | Cited 11 times

Online Publication Date: 22 July 2010

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We report on the results of testing the reference interaction site model (RISM) for the estimation of the hydration free energy of druglike molecules. The optimum model was selected after testing of different RISM free energy expressions combined with different quantum mechanics and empirical force-field methods of structure optimization and atomic partial charge calculation. The final model gave a systematic error with a standard deviation of 2.6 kcal/mol for a test set of 31 molecules selected from the SAMPL1 blind challenge set [ J. P. Guthrie, J. Phys. Chem. B 113, 4501 (2009) ]. After parametrization of this model to include terms for the excluded volume and the number of atoms of different types in the molecule, the root mean squared error for a test set of 19 molecules was less than 1.2 kcal/mol.
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87.15.np Dissolution
87.15.ag Quantum calculations
82.39.Rt Reactions in complex biological systems

A multistage ab initio quantum wavepacket dynamics formalism for electronic structure and dynamics in open systems

Alexander B. Pacheco and Srinivasan S. Iyengar

J. Chem. Phys. 133, 044105 (2010); http://dx.doi.org/10.1063/1.3463798 (15 pages) | Cited 4 times

Online Publication Date: 22 July 2010

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We propose a multistage quantum wavepacket dynamical treatment for the study of delocalized electronic systems as well as electron transport through donor-bridge-acceptor systems such as those found in molecular-wire/electrode networks. The full donor-bridge-acceptor system is treated through a rigorous partitioning scheme that utilizes judiciously placed offsetting absorbing and emitting boundary conditions. These facilitate a computationally efficient and potentially accurate treatment of the long-range coupling interactions between the bridge and donor/acceptor systems and the associated open system boundary conditions. Time-independent forms of the associated, partitioned equations are also derived. In the time-independent form corresponding to the bridge system, coupling to donor and acceptor, that is long-range interactions, is completely accounted. For the time-dependent study, the quantum dynamics of the electronic flux through the bridge-donor/acceptor interface is constructed using an accurate and efficient representation of the discretized quantum-mechanical free-propagator. A model for an electrode-molecular wire-electrode system is used to test the accuracy of the scheme proposed. Transmission probability is obtained directly from the probability density of the electronic flux in the acceptor region. Conductivity through the molecular wire is computed using a wavepacket flux correlation function.
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71.15.-m Methods of electronic structure calculations

Determination of spin Hamiltonians from projected single reference configuration interaction calculations. I. Spin 1/2 systems

A. Monari, D. Maynau, and J.-P. Malrieu

J. Chem. Phys. 133, 044106 (2010); http://dx.doi.org/10.1063/1.3458642 (11 pages) | Cited 1 time

Online Publication Date: 22 July 2010

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The most reliable wave-function based treatments of magnetic systems usually start from a complete active space self-consistent field calculation of the magnetic electrons in the magnetic orbitals, followed by extensive and expensive configuration interaction (CI) calculations. This second step, which introduces crucial spin polarization and dynamic correlation effects, is necessary to reach reliable values of the magnetic coupling constants. The computational cost of these approaches increases exponentially with the number of unpaired electrons. The single-determinantal unrestricted density functional Kohn–Sham calculations are computationally much simpler, and may provide reasonable estimates of these quantities, but their results are strongly dependent on the chosen exchange-correlation potential. The present work, which may be seen as an ab initio transcription of the unrestricted density functional theory technique, returns to the perturbative definition of the Heisenberg Hamiltonian as an effective Hamiltonian, and proposes a direct estimate of its diagonal energies through single reference CI calculations. The differences between these diagonal terms actually determine the entire Heisenberg Hamiltonian. The reference determinants must be vectors of the model space and the components on the other vectors of the model space are cancelled along the iterative process. The method is successfully tested on a series of bicentric and multicentric spin ½ systems. The projected single reference difference dedicated CI treatment is both accurate and of moderate cost. It opens the way to parameter-free calculations of large spin assemblies.
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75.10.Dg Crystal-field theory and spin Hamiltonians
75.10.Jm Quantized spin models, including quantum spin frustration
75.25.Dk Orbital, charge, and other orders, including coupling of these orders
75.50.Xx Molecular magnets

Embedding theory for excited states

Yuriy G. Khait and Mark R. Hoffmann

J. Chem. Phys. 133, 044107 (2010); http://dx.doi.org/10.1063/1.3460594 (6 pages) | Cited 5 times

Online Publication Date: 22 July 2010

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Using the technique of Perdew and Levy [Phys. Rev. B 31, 6264 (1985)] , it is shown that both the density function theory (DFT)-in-DFT and wave function theory (WFT)-in-DFT embedding approaches are formally correct in studying not only the ground state but also a subset of the excited states of the total system. Without further approximations, the DFT-in-DFT embedding approach results in a pair of coupled Euler–Lagrange equations. In contrast to DFT-in-DFT, the WFT-in-DFT approach is shown to ensure a systematic description of excited states if such states are mainly related to excitations within the embedded subsystem. Possible ways for the practical realization of the WFT-in-DFT approach for studying excited states are briefly discussed.
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31.15.E- Density-functional theory
31.15.vj Electron correlation calculations for atoms and ions: excited states

A computational study of ultrafast acid dissociation and acid-base neutralization reactions. I. The model

Patrick Maurer, Vibin Thomas, Ugo Rivard, and Radu Iftimie

J. Chem. Phys. 133, 044108 (2010); http://dx.doi.org/10.1063/1.3461162 (11 pages) | Cited 1 time

Online Publication Date: 23 July 2010

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Ultrafast, time-resolved investigations of acid-base neutralization reactions have recently been performed using systems containing the photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) and various Brønsted bases. Two conflicting neutralization mechanisms have been formulated by Mohammed et al. [Science 310, 83 (2005)] and Siwick et al. [J. Am. Chem. Soc. 129, 13412 (2007)] for the same acid-base system. Herein an ab initio molecular dynamics based computational model is formulated, which is able to investigate the validity of the proposed mechanisms in the general context of ground-state acid-base neutralization reactions. Our approach consists of using 2,4,6-tricyanophenol (exp. pKa ≅ 1) as a model for excited-state HPTS∗ (pKa ≅ 1.4) and carboxylate ions for the accepting base. We employ our recently proposed dipole-field/quantum mechanics (QM) treatment [ P. Maurer and R. Iftimie, J. Chem. Phys. 132, 074112 (2010) ] of the proton donor and acceptor molecules. This approach allows one to tune the free energy of neutralization to any desired value as well as model initial nonequilibrium hydration effects caused by a sudden increase in acidity, making it possible to achieve a more realistic comparison with experimental data than could be obtained via a full-QM treatment of the entire system. It is demonstrated that the dipole-field/QM model reproduces correctly key properties of the 2,4,6-tricyanophenol acid molecule including gas-phase proton dissociation energies and dipole moments, and condensed-phase hydration structure and pKa values.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
33.15.Fm Bond strengths, dissociation energies
82.20.-w Chemical kinetics and dynamics
31.15.A- Ab initio calculations
31.50.Df Potential energy surfaces for excited electronic states
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Bh General molecular conformation and symmetry; stereochemistry

Analyzing molecular static linear response properties with perturbed localized orbitals

Jochen Autschbach and Harry F. King

J. Chem. Phys. 133, 044109 (2010); http://dx.doi.org/10.1063/1.3455709 (10 pages)

Online Publication Date: 23 July 2010

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Perturbed localized molecular orbitals (LMOs), correct to first order in an applied static perturbation and consistent with a chosen localization functional, are calculated using analytic derivative techniques. The formalism is outlined for a general static perturbation and variational localization functionals. Iterative and (formally) single-step approaches are compared. The implementation employs an iterative sequence of 2×2 orbital rotations. The procedure is verified by calculations of molecular electric-field perturbations. Boys LMO contributions to the electronic static polarizability and the electric-field perturbation of the 〈r2〉 expectation value are calculated and analyzed for ethene, ethyne, and fluoroethene (H2CCHF). For ethene, a comparison is made with results from a Pipek–Mezey localization. The calculations show that a chemically intuitive decomposition of the calculated properties is possible with the help of the LMO contributions and that the polarizability contributions in similar molecules are approximately transferable.
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31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
31.15.ap Polarizabilities and other atomic and molecular properties
31.15.xt Variational techniques
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Solid-liquid surface free energy of Lennard-Jones liquid on smooth and rough surfaces computed by molecular dynamics using the phantom-wall method

Frédéric Leroy and Florian Müller-Plathe

J. Chem. Phys. 133, 044110 (2010); http://dx.doi.org/10.1063/1.3458796 (11 pages) | Cited 1 time

Online Publication Date: 23 July 2010

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Different model Lennard-Jones solid-liquid interfaces have been considered. In the systems, either the interaction strength between solid and liquid was varied, or the topography of the solid surface was modified. In all situations, the solid-liquid interfacial free energy variations with respect to a reference solid-liquid interface were quantified by means of a thermodynamic integration method [ F. Leroy et al., Macromol. Rapid Commun. 30, 864 (2009) ], referred to as the phantom-wall method. Additionally, the liquid-vapor surface free energy was determined. This result was combined with Young’s equation for contact angle calculations of cylindrical liquid droplets. It allowed us to show that the change in contact angle of a droplet placed on smooth solid surfaces with respect to solid-liquid interaction strength could be obtained by neglecting the solid-vapor surface free energy contribution when the solid-liquid interaction was weak. We also showed that the implementation of roughness by means of parallel grooves whose the density was varied could yield either higher or lower solid-liquid surface free energy, depending on the solid-liquid surface free energy of the smooth interface. Roughness led to lower surface free energy when the smooth surface had favorable interaction with the liquid, while it led to a higher surface free energy when the smooth surface had loose interactions with the liquid, though the effect was found to be weak. The consistency of the whole set of results, as well as agreement with the existing results on similar systems, shows the ability of the thermodynamic integration method employed here to capture the variation of interfacial thermodynamic quantities when modifying either the chemical nature or the topography of a solid surface in contact with a given liquid phase.
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68.03.Cd Surface tension and related phenomena
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.
65.20.-w Thermal properties of liquids
68.35.Md Surface thermodynamics, surface energies

Multi-Jastrow trial wavefunctions for electronic structure calculations with quantum Monte Carlo

Thomas Bouabça, Benoît Braïda, and Michel Caffarel

J. Chem. Phys. 133, 044111 (2010); http://dx.doi.org/10.1063/1.3457364 (18 pages) | Cited 1 time

Online Publication Date: 23 July 2010

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A new type of electronic trial wavefunction suitable for quantum Monte Carlo calculations of molecular systems is presented. In contrast with the standard Jastrow–Slater form built with a unique global Jastrow term, it is proposed to introduce individual Jastrow factors attached to molecular orbitals. Such a form is expected to be more physical since it allows to describe differently the local electronic correlations associated with various molecular environments (1s-core orbitals, 3d-magnetic orbitals, localized two-center σ-orbitals, delocalized π-orbitals, atomic lone pairs, etc.). In contrast with the standard form, introducing different Jastrow terms allows us to change the nodal structure of the wavefunction, a point which is important in the context of building better nodes for more accurate fixed-node diffusion Monte Carlo (FN-DMC) calculations. Another important aspect resulting from the use of local Jastrow terms is the possibility of defining and preoptimizing local and transferable correlated units for building complex trial wavefunctions from simple parts. The practical aspects associated with the computation of the intricate derivatives of the multi-Jastrow trial function are presented in detail. Some first illustrative applications for atoms of increasing size (O, S, and Cu) and for the potential energy curve and spectroscopic constants of the FH molecule are presented. In the case of the copper atom, the use of the multi-Jastrow form at the variational Monte Carlo level has allowed us to improve significantly the value of the total ground-state energy (about 75% of the correlation energy with only one determinant and three atomic orbital Jastrow factors). In the case of the FH molecule (fluorine hydride), it has been found that the multi-Jastrow nodes lead to an almost exact FN-DMC value of the dissociation energy [D0 = −140.7(4) kcal/mol instead of the estimated nonrelativistic Born–Oppenheimer exact value of −141.1], which is not the case with standard nodes, D0 = −138.3(4) kcal/mol.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
31.50.-x Potential energy surfaces

Fluctuating dynamics of nematic liquid crystals using the stochastic method of lines

A. K. Bhattacharjee, Gautam I. Menon, and R. Adhikari

J. Chem. Phys. 133, 044112 (2010); http://dx.doi.org/10.1063/1.3455206 (6 pages)

Online Publication Date: 27 July 2010

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We construct Langevin equations describing the fluctuations of the tensor order parameter Qαβ in nematic liquid crystals by adding noise terms to time-dependent variational equations that follow from the Ginzburg–Landau–de Gennes free energy. The noise is required to preserve the symmetry and tracelessness of the tensor order parameter and must satisfy a fluctuation-dissipation relation at thermal equilibrium. We construct a noise with these properties in a basis of symmetric traceless matrices and show that the Langevin equations can be solved numerically in this basis using a stochastic version of the method of lines. The numerical method is validated by comparing equilibrium probability distributions, structure factors, and dynamic correlations obtained from these numerical solutions with analytic predictions. We demonstrate excellent agreement between numerics and theory. This methodology can be applied to the study of phenomena where fluctuations in both the magnitude and direction of nematic order are important, as for instance, in the nematic swarms which produce enhanced opalescence near the isotropic-nematic transition or the problem of nucleation of the nematic from the isotropic phase.
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61.30.Jf Defects in liquid crystals
64.60.Q- Nucleation
05.40.Ca Noise
64.70.M- Transitions in liquid crystals
02.50.Ey Stochastic processes

Electronic excitation energy calculation by the fragment molecular orbital method with three-body effects

Mahito Chiba and Tetsuya Koido

J. Chem. Phys. 133, 044113 (2010); http://dx.doi.org/10.1063/1.3462247 (6 pages) | Cited 4 times

Online Publication Date: 27 July 2010

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A scheme for full quantum electronic excited state calculation is proposed that is based on the fragment molecular orbital (FMO) method with three-body effects. The accuracy and efficiency of this scheme is checked by calculating the excitation energy of hydrated formaldehyde and hydrated phenol. In all cases, three-body effects improved the excitation energy by the one- and two-body FMO methods with small computational cost, and the excitation energy approached more closely the full calculation value. The results also show that the three-body effects were relatively large and cannot be neglected.
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31.15.vj Electron correlation calculations for atoms and ions: excited states
31.50.Df Potential energy surfaces for excited electronic states
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
31.15.-p Calculations and mathematical techniques in atomic and molecular physics

Free energy calculation using molecular dynamics simulation combined with the three dimensional reference interaction site model theory. I. Free energy perturbation and thermodynamic integration along a coupling parameter

Tatsuhiko Miyata, Yasuhiro Ikuta, and Fumio Hirata

J. Chem. Phys. 133, 044114 (2010); http://dx.doi.org/10.1063/1.3462276 (15 pages) | Cited 2 times

Online Publication Date: 28 July 2010

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This article proposes a free energy calculation method based on the molecular dynamics simulation combined with the three dimensional reference interaction site model theory. This study employs the free energy perturbation (FEP) and the thermodynamic integration (TDI) along the coupling parameters to control the interaction potential. To illustrate the method, we applied it to a complex formation process in aqueous solutions between a crown ether molecule 18-Crown-6 (18C6) and a potassium ion as one of the simplest model systems. Two coupling parameters were introduced to switch the Lennard-Jones potential and the Coulomb potential separately. We tested two coupling procedures: one is a “sequential-coupling” to couple the Lennard-Jones interaction followed by the Coulomb coupling, and the other is a “mixed-coupling” to couple both the Lennard-Jones and the Coulomb interactions together as much as possible. The sequential-coupling both for FEP and TDI turned out to be accurate and easily handled since it was numerically well-behaved. Furthermore, it was found that the sequential-coupling had relatively small statistical errors. TDI along the mixed-coupling integral path was to be carried out carefully, paying attention to a numerical behavior of the integrand. The present model system exhibited a nonmonotonic behavior in the integrands for TDI along the mixed-coupling integral path and also showed a relatively large statistical error. A coincidence within a statistical error was obtained among the results of the free energy differences evaluated by FEP, TDI with the sequential-coupling, and TDI with the mixed-coupling. The last one is most attractive in terms of the computer power and is accurate enough if one uses a proper set of windows, taking the numerical behavior of the integrands into account. TDI along the sequential-coupling integral path would be the most convenient among the methods we tested, since it seemed to be well-balanced between the computational load and the accuracy. The numerical results reported in this article qualitatively agree with the experimental data for the potassium ion recognition by the 18C6 in aqueous solution.
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05.70.Ce Thermodynamic functions and equations of state
61.25.Em Molecular liquids
61.20.Ja Computer simulation of liquid structure

An efficient umbrella potential for the accurate calculation of free energies by molecular simulation

Di Wu

J. Chem. Phys. 133, 044115 (2010); http://dx.doi.org/10.1063/1.3464330 (9 pages) | Cited 1 time

Online Publication Date: 29 July 2010

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Umbrella sampling has been widely used to calculate free energies in many chemical and biological applications because it can effectively bridge the systems of interest and sample in the united phase space that is essential to yield accurate results. Many algorithms have implemented the idea of umbrella sampling that greatly improves the calculation of free energies. An efficient umbrella potential not only can connect the systems of interest, but also can lower the energetic barriers and facilitate the sampling over the relevant phase spaces. Here we present such an umbrella potential that is built on the equations of the weighted histogram analysis method. The proposed umbrella potential can facilitate the sampling of the important phase spaces of the systems of interest, which ensures the accurate calculation of free energies. We test this umbrella potential using a harmonic-model system, a water system, and a Lennard-Jones system. We demonstrate that this umbrella potential is effective in the circumstances when the systems of interest do not exhibit overlap in their phase spaces.
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05.70.Ce Thermodynamic functions and equations of state
31.10.+z Theory of electronic structure, electronic transitions, and chemical binding
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions

Hartree–Fock perturbative corrections for total and reaction energies

Jia Deng, Andrew T. B. Gilbert, and Peter M. W. Gill

J. Chem. Phys. 133, 044116 (2010); http://dx.doi.org/10.1063/1.3463800 (6 pages) | Cited 5 times

Online Publication Date: 30 July 2010

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We have performed an assessment of the Hartree–Fock perturbative correction (HFPC) on a large and diverse set of molecules and reactions. Errors in both absolute and reaction energies with respect to converged secondary basis Hartree–Fock results are reported for a wide spectrum of primary/secondary basis set combinations. These results show that using an adequate primary basis, HFPC can accurately reproduce secondary basis energies at a substantially reduced cost. Comparisons of HFPC with the related dual basis Hartree–Fock (DBHF) scheme are also made for several molecules and target secondary basis sets. Our results indicate that HFPC is faster and more accurate than DBHF for approaching triple-ζ basis sets. For quadruple-ζ secondary basis sets, HFPC is capable of yielding more accurate energies at a marginally increased cost over DBHF.
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31.15.xr Self-consistent-field methods
82.20.-w Chemical kinetics and dynamics
31.15.xp Perturbation theory

Transferability of anharmonic force fields in simulations of molecular vibrations

Václav Parchaňský and Petr Bouř

J. Chem. Phys. 133, 044117 (2010); http://dx.doi.org/10.1063/1.3464759 (10 pages) | Cited 1 time

Online Publication Date: 30 July 2010

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Accurate simulations of vibrational molecular spectra require precise molecular force fields, at least with cubic and quartic anharmonic corrections beyond the harmonic limits. Generation of such force field terms becomes computationally prohibitive for larger molecules. In this work, an alternate possibility is explored, where approximate anharmonic force field components are obtained from molecular fragments. Transferability properties of the cubic and incomplete quartic fields are discussed and tested on model oligoproline molecules. Automatic transfer schemes including cubic, two and three atomic quartic force constants are developed and implemented. The results indicate that the main vibrational interactions in molecules are local and the anharmonic constants are mostly well amendable to the transfer. Exact anharmonic normal mode force fields of larger molecules compared very well to those obtained from smaller molecular parts. The most important changes in vibrational spectra caused by the anharmonic interactions could be reproduced with two and three atomic force field terms. The transfer scheme thus provides molecular anharmonic force fields without a significant loss of accuracy and brings significant savings of computer time and memory needed to generate molecular vibrational energies and spectra.
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33.20.Tp Vibrational analysis
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Dissociation energy of the ground state of NaH

Hsien-Yu Huang, Tsai-Lien Lu, Thou-Jen Whang, Yung-Yung Chang, and Chin-Chun Tsai

J. Chem. Phys. 133, 044301 (2010); http://dx.doi.org/10.1063/1.3458914 (7 pages) | Cited 1 time

Online Publication Date: 22 July 2010

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The dissociation energy of the ground state of NaH was determined by analyzing the observed near dissociation rovibrational levels. These levels were reached by stimulated emission pumping and fluorescence depletion spectroscopy. A total of 114 rovibrational levels in the ranges 9 ≤ v″ ≤ 21 and 1 ≤ J″ ≤ 14 were assigned to the X 1Σ+ state of NaH. The highest vibrational level observed was only about 40 cm−1 from the dissociation limit in the ground state. One quasibound state, above the dissociation limit and confined by the centrifugal barrier, was observed. Determining the vibrational quantum number at dissociation vD from the highest four vibrational levels yielded the dissociation energy De = 15 815±5 cm−1. Based on new observations and available data, a set of Dunham coefficients and the rotationless Rydberg–Klein–Rees curve were constructed. The effective potential curve and the quasibound states were discussed.
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33.15.Fm Bond strengths, dissociation energies
33.20.Vq Vibration-rotation analysis
33.80.Be Level crossing and optical pumping
33.20.Tp Vibrational analysis
33.80.Gj Diffuse spectra; predissociation, photodissociation
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
31.50.Df Potential energy surfaces for excited electronic states
03.65.Ge Solutions of wave equations: bound states

Fivefold differential cross sections for ground-state ionization of aligned H2 by electron impact

Arne Senftleben, Ola Al-Hagan, Thomas Pflüger, Xueguang Ren, Don Madison, Alexander Dorn, and Joachim Ullrich

J. Chem. Phys. 133, 044302 (2010); http://dx.doi.org/10.1063/1.3457155 (7 pages) | Cited 2 times

Online Publication Date: 22 July 2010

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We discuss the ionization of aligned hydrogen molecules into their ionic ground state by 200 eV electrons. Using a reaction microscope, the complete electron scattering kinematics is imaged over a large solid angle. Simultaneously, the molecular alignment is derived from postcollision dissociation of the residual ion. It is found that the ionization cross section is maximized for small angles between the internuclear axis and the momentum transfer. Fivefold differential cross sections (5DCSs) reveal subtle differences in the scattering process for the distinct alignments. We compare our observations with theoretical 5DCSs obtained with an adapted molecular three-body distorted wave model that reproduces most of the results, although discrepancies remain.
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34.80.Gs Molecular excitation and ionization

Vibrationally resolved photoelectron imaging of gold hydride cluster anions: AuH− and Au2H−

Xia Wu, Zhengbo Qin, Hua Xie, Ran Cong, Xiaohu Wu, Zichao Tang, and Hongjun Fan

J. Chem. Phys. 133, 044303 (2010); http://dx.doi.org/10.1063/1.3456373 (5 pages) | Cited 1 time

Online Publication Date: 23 July 2010

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Photoelectron spectra and angular distributions in photodetachment of gold hydride anions AuH− and Au2H− have been obtained using photoelectron velocity-map imaging. Both the images exhibit vibrationally resolved ground state transitions. The adiabatic electron affinities of AuH and Au2H are measured to be 0.758(20) and 3.437(3) eV, respectively. Franck–Condon analyses of the AuH spectra determined that the equilibrium bond length of the ground state of AuH− is 1.597(6) math. The photoelectron images of Au2H− show a vibrational progression of 148(4) cm−1 assigned to the Au–Au stretching mode at the ground state. Ab initio calculation results are in excellent agreement with the experimental results. For the ground state of Au2H, a new bent Au–Au–H structure with the angle of 131° is suggested. Moreover, energy-dependent photoelectron anisotropy parameters are also reported and discussed.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.80.Eh Autoionization, photoionization, and photodetachment
33.60.+q Photoelectron spectra
33.15.Dj Interatomic distances and angles
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
31.15.A- Ab initio calculations
36.40.Wa Charged clusters

Sound-driven fluid dynamics in pressurized carbon dioxide

Maikel M. van Iersel, Robert Mettin, Nieck E. Benes, Dirk Schwarzer, and Jos T. F. Keurentjes

J. Chem. Phys. 133, 044304 (2010); http://dx.doi.org/10.1063/1.3463444 (4 pages)

Online Publication Date: 23 July 2010

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Using high-speed visualization we demonstrate that ultrasound irradiation of pressurized carbon dioxide (CO2) induces phenomena that do not occur in ordinary liquids at ambient conditions. For a near-critical mixture of CO2 and argon, sonication leads to extremely fast local phase separation, in which the system enters and leaves the two-phase region with the frequency of the imposed sound field. This phase transition can propagate with the speed of sound, but can also be located at fixed positions in the case of a standing sound wave. Sonication of a vapor-liquid interface creates a fine dispersion of liquid and vapor, irrespective whether the ultrasound horn is placed in the liquid or the vapor phase. In the absence of an interface, sonication of the liquid leads to ejection of a macroscopic vapor phase from the ultrasound horn with a velocity of several meters per second in the direction of wave propagation. The findings reported here potentially provide a tunable and noninvasive means for enhancing mass and heat transfer in high-pressure fluids.
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62.60.+v Acoustical properties of liquids
64.70.F- Liquid-vapor transitions
62.50.-p High-pressure effects in solids and liquids

Use of complex configuration interaction calculations and the stationary principle for the description of metastable electronic states of HCl−

Michael Honigmann, Heinz-Peter Liebermann, and Robert J. Buenker

J. Chem. Phys. 133, 044305 (2010); http://dx.doi.org/10.1063/1.3467885 (15 pages) | Cited 1 time

Online Publication Date: 26 July 2010

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The complex multireference single- and double-excitation configuration interaction method has been employed to compute potential curves for the anion of the hydrogen chloride molecule. First, conventional CI calculations with real basis functions have been carried out to determine the potential curves of both HCl and its anion over a large range of internuclear distance. It is shown that adding basis functions with very small exponents leads to sharply avoided crossings for the HCl− potentials that greatly complicate the search for resonance states thought to be responsible for features observed in electron collision experiments. By limiting the number of such diffuse-type functions it is possible to describe resonance states at a highly correlated level and still account for their interaction with the continuum in which they are embedded. In the present study of the HCl− anion the complex basis function technique of Moiseyev–Corcoran and McCurdy–Resigno is employed to calculate the energy positions and line-widths of the resonance states. Two states of 2Σ+ symmetry are calculated which have potentials that have significantly different shapes than that of the neutral ground state and thus contribute to the cross section for vibrational excitation of the neutral HCl molecule induced by low-energy electron collisions. The lower of these (1 2Σ+) correlates smoothly with the bound anionic ground state at large internuclear distances and is seen to be responsible for the sharp peaks observed in the low-energy region of the spectrum. The upper state (3 2Σ+) has a much larger bond length and is assigned to the broad bands observed with a maximum in the 2.5–3.0 eV range. The present calculations thus stand in contradiction to earlier claims that the above peaks are caused by so-called virtual states without a definite autoionization lifetime.
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31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
31.15.vj Electron correlation calculations for atoms and ions: excited states
34.80.Bm Elastic scattering
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Dj Interatomic distances and angles

Structure and spectroscopy of ground and excited states of LiYb

P. Zhang, H. R. Sadeghpour, and A. Dalgarno

J. Chem. Phys. 133, 044306 (2010); http://dx.doi.org/10.1063/1.3462245 (10 pages) | Cited 2 times

Online Publication Date: 28 July 2010

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Multireference configuration interaction and coupled cluster calculations have been carried out to determine the potential energy curves for the ground and low-lying excited states of the LiYb molecule. The scalar relativistic effects have been included by means of the Douglas–Kroll Hamiltonian and effective core potential and the spin-orbit couplings have been evaluated by the full microscopic Breit–Pauli operator. The LiYb permanent dipole moment, static dipole polarizability, and Franck–Condon factors have been determined. Perturbations of the vibrational spectrum due to nonadiabatic interactions are discussed.
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31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
31.15.bw Coupled-cluster theory
31.50.Df Potential energy surfaces for excited electronic states
31.50.Bc Potential energy surfaces for ground electronic states
31.15.aj Relativistic corrections, spin-orbit effects, fine structure; hyperfine structure
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
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