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14 Aug 2006

Volume 125, Issue 6, Articles (06xxxx)

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Enhancement of molecular modes by electronically resonant multipulse excitation: Further progress towards mode selective chemistry

Jürgen Hauer, Tiago Buckup, and Marcus Motzkus

J. Chem. Phys. 125, 061101 (2006); http://dx.doi.org/10.1063/1.2243273 (3 pages) | Cited 21 times

Online Publication Date: 8 August 2006

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We show that molecular vibrations induced by resonant excitation pulses can be enhanced by pulse trains, compared to Fourier-limited pulses of equal pulse energy. As a proof-of-principle, a low frequency mode of Nile Blue at 600 cm−1 is observed and amplified in a pump and probe experiment. In addition to previous experiments in our group, an increased population transfer to the excited electronic state is identified as an important element of the underlying physical mechanism. These results suggest an enhancement on the level of individual molecules rather than a macroscopic effect.
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33.80.Be Level crossing and optical pumping
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.50.Df Potential energy surfaces for excited electronic states

Rotational dynamics of a diatomic solute in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate

Youngseon Shim, Daun Jeong, M. Y. Choi, and Hyung J. Kim

J. Chem. Phys. 125, 061102 (2006); http://dx.doi.org/10.1063/1.2232303 (4 pages) | Cited 8 times

Online Publication Date: 8 August 2006

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Reorientational time correlation functions Cl(t)( ≡ 〈Pl[cos θ(t)]〉) for a diatomic solute in 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI+PF6−) are analyzed via molecular dynamics computer simulations, where 〈…〉 denotes an equilibrium ensemble average, Pl the lth order Legendre polynomial and θ(t) the angle between the solute orientation at time t and its initial direction. Overall results are indicative of heterogeneous dynamics in EMI+PF6−. For a small nondipolar solute, Cl(t) are well-described as stretched exponential functions in wide time ranges. One striking feature is that after rapid initial relaxation, C2(t) decays more slowly than C1(t). As a result, the correlation time associated with the former is considerably longer than that with the latter. This is ascribed to solvent structural fluctuations, which allow large-amplitude solute rotations. As the solute size grows, relaxation of Cl(t) approaches exponential decay.
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61.25.Em Molecular liquids
61.20.Lc Time-dependent properties; relaxation
61.20.Ja Computer simulation of liquid structure
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back to top Theoretical Methods and Algorithms

The electronic ground-state energy problem: A new reduced density matrix approach

Eric Cancès, Gabriel Stoltz, and Mathieu Lewin

J. Chem. Phys. 125, 064101 (2006); http://dx.doi.org/10.1063/1.2222358 (5 pages) | Cited 35 times

Online Publication Date: 8 August 2006

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We present here a formulation of the electronic ground-state energy in terms of the second order reduced density matrix, using a duality argument. It is shown that the computation of the ground-state energy reduces to the search of the projection of some two-electron reduced Hamiltonian on the dual cone of N-representability conditions. Some numerical results validate the approach, both for equilibrium geometries and for the dissociation curve of N2.
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31.15.E- Density-functional theory
33.15.Fm Bond strengths, dissociation energies

The origin of molecular distortions: A proposed experimental test

R. Englman and T. Vértesi

J. Chem. Phys. 125, 064102 (2006); http://dx.doi.org/10.1063/1.2236111 (7 pages) | Cited 4 times

Online Publication Date: 8 August 2006

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Bersuker and co-workers [ Vibronic Interactions in Molecules and Crystals (Springer-Verlag, Berlin, 1989); Chem. Rev. 101, 1067 (2001); The Jahn-Teller Effect (Cambridge University Press, Cambridge, England, 2005); J. Chem. Phys. 124, 044321 (2006) ] showed that the source of distortions from high symmetry configurations in molecules is vibrational-electronic (vibronic). It is here suggested that if one induces transitions by two consecutive operations (vibrational and electronic) performed in a given and in the reverse order, then the vibronic scenario gives different probabilities for the two orders, while an alternative scenario gives the same probability. The entanglement measure of the vibronic description is noted, and the broader aspect of the suggestion for experimentally testing state entanglements is discussed.
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33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Mt Rotation, vibration, and vibration-rotation constants

Generalized coarse-grained model based on point multipole and Gay-Berne potentials

Pavel A. Golubkov and Pengyu Ren

J. Chem. Phys. 125, 064103 (2006); http://dx.doi.org/10.1063/1.2244553 (11 pages) | Cited 16 times

Online Publication Date: 8 August 2006

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This paper presents a general coarse-grained molecular mechanics model based on electric point multipole expansion and Gay-Berne [J. Chem. Phys. 74, 3316 (1981) ] potential. Coarse graining of van der Waals potential is achieved by treating molecules as soft uniaxial ellipsoids interacting via a generalized anisotropic Gay-Berne function. The charge distribution is represented by point multipole expansion, including point charge, dipole, and quadrupole moments placed at the center of mass. The Gay-Berne and point multipole potentials are combined in the local reference frame defined by the inertial frame of the all-atom counterpart. The coarse-grained model has been applied to rigid-body molecular dynamics simulations of molecular liquids including benzene and methanol. The computational efficiency is improved by several orders of magnitude, while the results are in reasonable agreement with all-atom models and experimental data. We also discuss the implications of using point multipole for polar molecules capable of hydrogen bonding and the applicability of this model to a broad range of molecular systems including highly charged biopolymers.
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61.20.Ja Computer simulation of liquid structure
34.20.Gj Intermolecular and atom-molecule potentials and forces
61.25.Em Molecular liquids

Using a family of dividing surfaces normal to the minimum energy path for quantum instanton rate constants

Yimin Li and William H. Miller

J. Chem. Phys. 125, 064104 (2006); http://dx.doi.org/10.1063/1.2220567 (8 pages) | Cited 2 times

Online Publication Date: 8 August 2006

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One of the outstanding issues in the quantum instanton (QI) theory (or any transition-state-type theory) for thermal rate constants of chemical reactions is the choice of an appropriate “dividing surface” (DS) that separates reactants and products. (In the general version of the QI theory, there are actually two dividing surfaces involved.) This paper shows one simple and general way for choosing DSs for use in QI theory, namely, using the family of (hyper) planes normal to the minimum energy path on the potential energy surface at various distances s along it. Here the reaction coordinate is not one of the dynamical coordinates of the system (which will in general be the Cartesian coordinates of the atoms), but rather simply a parameter which specifies the DS. It is also shown how this idea can be implemented for an N atom system in three-dimensional space in a way that preserves overall translational and rotational invariance. Numerical application to a simple system (the collinear H+H2 reaction) is presented to illustrate the procedure.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Kh Potential energy surfaces for chemical reactions

Wigner molecules: Natural orbitals of strongly correlated two-electron harmonium

Jerzy Cioslowski and Marcin Buchowiecki

J. Chem. Phys. 125, 064105 (2006); http://dx.doi.org/10.1063/1.2222360 (5 pages) | Cited 4 times

Online Publication Date: 10 August 2006

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Explicit asymptotic expressions for natural orbitals and their occupancies are derived for the harmonium atom at the strong-correlation limit at which the confinement strength ω tends to zero. Unlike in systems with moderate correlation effects, the occupancies at the ω→0 limit (derived from occupation amplitudes with alternating sign patterns) are vanishingly small and asymptotically independent of the angular momentum, forming a geometric progression with the scale factor proportional to ω1/3 and the common ratio of ca. 0.0186. The radial components of the natural orbitals are given by products of polynomials and Gaussian functions that, as expected, peak at approximately half of the equilibrium interelectron distance.
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71.27.+a Strongly correlated electron systems; heavy fermions
31.10.+z Theory of electronic structure, electronic transitions, and chemical binding

Wigner molecules: The strong-correlation limit of the three-electron harmonium

Jerzy Cioslowski and Katarzyna Pernal

J. Chem. Phys. 125, 064106 (2006); http://dx.doi.org/10.1063/1.2222361 (4 pages) | Cited 11 times

Online Publication Date: 10 August 2006

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At the strong-correlation limit, electronic states of the three-electron harmonium atom are described by asymptotically exact wave functions given by products of distinct Slater determinants and a common Gaussian factor that involves interelectron distances and the center-of-mass position. The Slater determinants specify the angular dependence and the permutational symmetry of the wave functions. As the confinement strength becomes infinitesimally small, the states of different spin multiplicities become degenerate, their limiting energy reflecting harmonic vibrations of the electrons about their equilibrium positions. The corresponding electron densities are given by products of angular factors and a Gaussian function centered at the radius proportional to the interelectron distance at equilibrium. Thanks to the availability of both the energy and the electron density, the strong-correlation limit of the three-electron harmonium is well suited for testing of density functionals.
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71.27.+a Strongly correlated electron systems; heavy fermions
31.10.+z Theory of electronic structure, electronic transitions, and chemical binding

Mean field linear response within the elimination of the small component formalism to evaluate relativistic effects on magnetic properties

P. G. Roura, J. I. Melo, M. C. Ruiz de Azúa, and C. G. Giribet

J. Chem. Phys. 125, 064107 (2006); http://dx.doi.org/10.1063/1.2244572 (10 pages) | Cited 4 times

Online Publication Date: 10 August 2006

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The linear response within the elimination of the small component formalism is aimed at obtaining the leading order relativistic corrections to magnetic molecular properties in the context of the elimination of the small component approximation. In the present work we extend the method in order to include two-body effects in the form of a mean field one-body operator. To this end we consider the four-component Dirac-Hartree-Fock operator as the starting point in the evaluation of the second order relativistic expression of magnetic properties. The approach thus obtained is the fully consistent leading order approximation of the random phase approximation four-component formalism. The mean field effect on the relativistic corrections to both the diamagnetic and paramagnetic terms of magnetic properties taking into account both the Coulomb and Breit two-body interactions is considered.
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33.25.+k Nuclear resonance and relaxation
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
31.15.xr Self-consistent-field methods

High-accuracy extrapolated ab initio thermochemistry. II. Minor improvements to the protocol and a vital simplification

Yannick J. Bomble, Juana Vázquez, Mihály Kállay, Christine Michauk, Péter G. Szalay, Attila G. Császár, Jürgen Gauss, and John F. Stanton

J. Chem. Phys. 125, 064108 (2006); http://dx.doi.org/10.1063/1.2206789 (8 pages) | Cited 63 times

Online Publication Date: 10 August 2006

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The recently developed high-accuracy extrapolated ab initio thermochemistry method for theoretical thermochemistry, which is intimately related to other high-precision protocols such as the Weizmann-3 and focal-point approaches, is revisited. Some minor improvements in theoretical rigor are introduced which do not lead to any significant additional computational overhead, but are shown to have a negligible overall effect on the accuracy. In addition, the method is extended to completely treat electron correlation effects up to pentuple excitations. The use of an approximate treatment of quadruple and pentuple excitations is suggested; the former as a pragmatic approximation for standard cases and the latter when extremely high accuracy is required. For a test suite of molecules that have rather precisely known enthalpies of formation {as taken from the active thermochemical tables of Ruscic and co-workers [Lecture Notes in Computer Science, edited by M. Parashar (Springer, Berlin, 2002), Vol. 2536, pp. 25–38 ; J. Phys. Chem. A 108, 9979 (2004)] }, the largest deviations between theory and experiment are 0.52, −0.70, and 0.51 kJ mol−1 for the latter three methods, respectively. Some perspective is provided on this level of accuracy, and sources of remaining systematic deficiencies in the approaches are discussed.
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82.60.Cx Enthalpies of combustion, reaction, and formation

Hybrid exchange-correlation functional for core, valence, and Rydberg excitations: Core-valence-Rydberg B3LYP

Ayako Nakata, Yutaka Imamura, and Hiromi Nakai

J. Chem. Phys. 125, 064109 (2006); http://dx.doi.org/10.1063/1.2227379 (9 pages) | Cited 25 times

Online Publication Date: 10 August 2006

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The core-valence-Rydberg Becke’s three-parameter exchange (B3)+Lee-Yang-Parr (LYP) correlation functional (CVR-B3LYP) is proposed as a means to improve descriptions of Rydberg excitations of core-valence B3LYP (CV-B3LYP). CV-B3LYP describes excitations from both core and occupied valence orbitals to unoccupied valence orbitals with high accuracy but fails to describe those to Rydberg orbitals. CVR-B3LYP, which adopts the appropriate portions of Hartree-Fock exchange for unoccupied valence and Rydberg regions separately, overcomes the disadvantage of CV-B3LYP. Numerical assessment confirms that time-dependent density functional theory calculations with CVR-B3LYP succeed in describing not only core excitations but also Rydberg excitations with reasonable accuracy.
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31.15.E- Density-functional theory
31.50.Df Potential energy surfaces for excited electronic states

Relative Boltzmann entropy, evolution equations for fluctuations of thermodynamic intensive variables, and a statistical mechanical representation of the zeroth law of thermodynamics

Byung Chan Eu

J. Chem. Phys. 125, 064110 (2006); http://dx.doi.org/10.1063/1.2208360 (12 pages)

Online Publication Date: 10 August 2006

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Generalized thermodynamics or extended irreversible thermodynamics presumes the existence of thermodynamic intensive variables (e.g., temperature, pressure, chemical potentials, generalized potentials) even if the system is removed from equilibrium. It is necessary to properly understand the nature of such intensive variables and, in particular, of their fluctuations, that is, their deviations from those defined in the extended irreversible thermodynamic sense. The meaning of temperature is examined by means of a kinetic theory of macroscopic irreversible processes to assess the validity of the generalized (or extended) thermodynamic method applied to nonequilibrium phenomena. The Boltzmann equation is used for the purpose. Since the relative Boltzmann entropy has been known to be intimately related to the evolution of the aforementioned fluctuations in the intensive thermodynamic variables, we derive the evolution equations for such fluctuations of intensive variables to lay the foundation for investigating the physical implications and evolution of the relative Boltzmann entropy, so that the range of validity of the thermodynamic theory of irreversible processes can be elucidated. Within the framework of this work, we examine a special case of the evolution equations for the aforementioned fluctuations of intensive variables, which also facilitate investigation of the molecular theory meaning of the zeroth law of thermodynamics. We derive an evolution equation describing the relaxation of temperature fluctuations from its local value and present a formula for the temperature relaxation time.
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05.70.Ce Thermodynamic functions and equations of state
05.70.Ln Nonequilibrium and irreversible thermodynamics
05.60.-k Transport processes
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
05.20.Dd Kinetic theory

Extensions of r12 corrections to CC2-R12 for excited states

Christian Neiss, Christof Hättig, and Wim Klopper

J. Chem. Phys. 125, 064111 (2006); http://dx.doi.org/10.1063/1.2335443 (7 pages) | Cited 17 times

Online Publication Date: 11 August 2006

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As known since about two decades, R12 methods, which include terms linear in the interelectronic distance r12 in the wave function, improve substantially the basis set convergence of the ground state correlation energy. In a previous study, however, it was found that the same approach does not give a similar systematic improvement if applied to excited states in the framework of coupled cluster response theory. In the present work, we examine the reason for this behavior and show that the inclusion of additional orbitals in the construction of the r12 pair functions leads to an enhanced basis set convergence (and thus a balanced description) also for the excited states.
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31.15.bw Coupled-cluster theory
31.15.vj Electron correlation calculations for atoms and ions: excited states

Multidimensional reactive scattering with quantum trajectories: Dynamics with Morse vibrational modes

Dmytro Babyuk and Robert E. Wyatt

J. Chem. Phys. 125, 064112 (2006); http://dx.doi.org/10.1063/1.2218335 (7 pages) | Cited 9 times

Online Publication Date: 11 August 2006

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The reactive scattering of a wave packet is studied by the quantum trajectory method for a model system with up to 25 Morse vibrational modes. The equations of motion are formulated in curvilinear reaction path coordinates with the restriction to a planar reaction path. Spatial derivatives are evaluated by the least squares method using contracted basis sets. Dynamical results, including trajectory evolution and time-dependent reaction probabilities, are presented and analyzed. For the case of one Morse vibrational mode, the results are in good agreement with those derived through direct numerical integration of the time-dependent Schrödinger equation.
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82.20.-w Chemical kinetics and dynamics
82.30.-b Specific chemical reactions; reaction mechanisms

Origin invariant approaches to the calculation of two-photon circular dichroism

Antonio Rizzo, Branislav Jansík, Thomas Bondo Pedersen, and Hans Ågren

J. Chem. Phys. 125, 064113 (2006); http://dx.doi.org/10.1063/1.2244562 (11 pages) | Cited 6 times

Online Publication Date: 15 August 2006

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The origin dependence of the expression of two-photon circular dichroism in the length formulation employed by the authors in a recent computational study [ B. Jansík et al., Chem. Phys. Lett. 414, 461 (2005) ] is discussed in detail, and some inherently origin invariant alternative formulations are introduced. Extensive computational tests on a small reference chiral system, namely, a chiral form of H2O2, are performed at the density functional theory (DFT)/B3LYP level of theory with large diffuse correlation consistent basis sets. The results indicate that the velocity formulation originally proposed by Tinoco, Jr. [J. Chem. Phys. 62, 1006 (1975)] provides the most convenient approach for an origin invariant calculation of two-photon circular dichroism.
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33.55.+b Optical activity and dichroism
33.80.Wz Other multiphoton processes
31.15.E- Density-functional theory
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

The nuclear electric quadrupole moment of antimony from the molecular method

Roberto L. A. Haiduke, Albérico B. F. da Silva, and Lucas Visscher

J. Chem. Phys. 125, 064301 (2006); http://dx.doi.org/10.1063/1.2234369 (5 pages) | Cited 7 times

Online Publication Date: 8 August 2006

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Relativistic Dirac-Coulomb (DC) Hartree-Fock calculations are employed to obtain the analytic electric field gradient (EFG) on the antimony nucleus in the SbN, SbP, SbF, and SbCl molecules. The electronic correlation contribution to the EFGs is included with the DC-CCSD(T) and DC-CCSD-T approaches, also in the four-component framework, using a finite-difference method. The total EFG results, along with the experimental nuclear quadrupole coupling constants from microwave spectroscopy, allow to derive the nuclear quadrupole moments of math and math, respectively, as −543(11) and −692(14) mb.
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31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
31.15.xr Self-consistent-field methods
31.15.bw Coupled-cluster theory
33.25.+k Nuclear resonance and relaxation
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

High resolution laser induced fluorescence spectroscopy of the [18.8] math−X math (0,0) band of cobalt monofluoride

Timothy C. Steimle, Tongmei Ma, Allan G. Adam, William D. Hamilton, and Anthony J. Merer

J. Chem. Phys. 125, 064302 (2006); http://dx.doi.org/10.1063/1.2221688 (9 pages) | Cited 6 times

Online Publication Date: 8 August 2006

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The fine and hyperfine interaction parameters in the [18.8] math (υ = 0) and X math (υ = 0) states of cobalt monofluoride, CoF, have been determined from an analysis of high-resolution laser induced fluorescence spectra of the [18.8] math−X math and [18.8] math−X math band systems. The previously reported pure rotational transitions of the X math(υ = 0) state [ T. Okabayashi and M. Tanimoto, J. Mol. Spectrosc. 221, 149 (2003) ] were included in the data set. The hyperfine parameters for math (I = 7/2) and math (I = 1/2) have been interpreted using atomic data together with a proposed molecular orbital description for the [18.8]math and X math states. A comparison of the hyperfine parameters in the X math state of cobalt monohydride, CoH, with those of the X math state of CoF reveals that the bonding in the two molecules is significantly different. It is shown that, in a situation where the Ω substates of a multiplet degenerate electronic state are analyzed separately, the Fermi contact parameter b can be determined with fair accuracy from the apparent centrifugal distortion of the hyperfine structure.
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33.50.Dq Fluorescence and phosphorescence spectra
31.30.Gs Hyperfine interactions and isotope effects
33.80.-b Photon interactions with molecules
33.15.Pw Fine and hyperfine structure

A new approximation for atom-diatom rotational-relaxation cross sections

Alan S. Dickinson

J. Chem. Phys. 125, 064303 (2006); http://dx.doi.org/10.1063/1.2243296 (6 pages) | Cited 1 time

Online Publication Date: 9 August 2006

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A semiclassical approximation to the S matrix of the infinite-order-sudden approximation is introduced. This is employed to yield for the energy-transfer effective cross section a purely classical approximation, analogous to the Mason-Monchick approximation [ J. Chem. Phys. 36, 1622 (1962) ] for traditional collision integrals. Constraints on energy and on angular momentum transfer are included. Numerical evaluation of this new approximation can readily be performed alongside that for traditional collision integrals. The new result is tested against full classical trajectory calculations for six potential energy surfaces for the collision systems H–N2, He–N2, He–CO, and Ar–CO2. Differences of no more than 15% from the classical trajectory calculations have been obtained.
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34.50.Ez Rotational and vibrational energy transfer
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions

Direct dynamics study on the hydrogen abstraction reactions N2H4+R→N2H3+RH (R = NH2,CH3)

Qian Shu Li and Xin Zhang

J. Chem. Phys. 125, 064304 (2006); http://dx.doi.org/10.1063/1.2217949 (12 pages) | Cited 3 times

Online Publication Date: 9 August 2006

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We present a direct ab initio dynamics study on the hydrogen abstraction reactions N2H4+R→N2H3+RH (R = NH2,CH3), which are predicted to have six possible reaction channels for NH2 abstraction and four for CH3 abstraction caused by the different N2H4 isomers and various attacking orientations of foreign radical to N2H4. The structures and frequencies at the stationary points and the points along the minimum energy paths (MEPs) of all reaction channels are obtained at the UMP2(full)/6-31+G(d,p) level of theory. Energetic information of stationary points and the points along the MEPs is further refined by means of MC-QCISD method. The rate constants of these channels are calculated using the improved canonical variational transition-state theory with the small-curvature tunneling correction (ICVT/SCT) method. The calculated results show that the favorable reaction channels are channels (n1) and (n4) as well as (c1) and (c3) (refer to Scheme 1) in the whole temperature range. The total ICVT/SCT rate constants of all channels for the two reactions at the MC-QCISD//UMP2(full)/6-31+G(d,p) level are both in good agreement with the available experimental data, and corresponding three-parameter expressions of kICVT/SCT in 220–3000 K are fitted as 6.46×10−15(T/298)3.60 exp(−386/T) cm3 mol−1 s−1 for NH2 abstraction and 1.04×10−14(T/298)4.00 exp(−2037/T) cm3 mol−1 s−1 for CH3 abstraction. Additionally, the long range interaction between the H atom of X–H bond in foreign radicals and the lone pair on the nonreactive N atom of the transition states is further discussed to explain the various transition-state numbers of the two similar hydrogen abstraction reactions.
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82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Db Transition state theory and statistical theories of rate constants
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Qt Isomerization and rearrangement

Interaction potentials for Br−–Rg (Rg = He–Rn): Spectroscopy and transport coefficients

Alexei A. Buchachenko, Jacek Kłos, Małgorzata M. Szczȩśniak, Grzegorz Chałasiński, Benjamin R. Gray, Timothy G. Wright, Erin L. Wood, Larry A. Viehland, and Enming Qing

J. Chem. Phys. 125, 064305 (2006); http://dx.doi.org/10.1063/1.2244571 (12 pages) | Cited 9 times

Online Publication Date: 10 August 2006

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High-level ab initio CCSD(T) calculations are performed in order to obtain accurate interaction potentials for the Br− anion interacting with each rare gas (Rg) atom. For the Rg atoms from He to Ar, two approaches are taken. The first one implements a relativistic core potential and an aug-cc-pVQZ basis set for bromine, an aug-cc-pV5Z basis set for Rg, and a set of bond functions placed at the midpoint of the Rg–Br distance. The second one uses the all-electron approximation with aug-cc-pV5Z bases further augmented by an extra diffuse function in each shell. Comparison reveals close similarity between both sets of results, so for Rg atoms from Kr to Rn only the second approach is exploited. Calculated potentials are assessed against the previous empirical, semiempirical, and ab initio potentials, and against available beam scattering data, zero electron kinetic energy spectroscopic data, and various sets of the measured ion mobilities and diffusion coefficients. This multiproperty analysis leads to the conclusion that the present potentials are consistently good for the whole series of Br−–Rg pairs over the whole range of internuclear distances covered.
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34.20.Cf Interatomic potentials and forces
31.15.A- Ab initio calculations
31.15.bw Coupled-cluster theory

Structure and stability of Al13Hn (n = 1–13) clusters: Exceptional stability of Al13H13

Jaehoon Jung and Young-Kyu Han

J. Chem. Phys. 125, 064306 (2006); http://dx.doi.org/10.1063/1.2244568 (3 pages) | Cited 13 times

Online Publication Date: 10 August 2006

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We have performed density functional calculations for the structure and stability of Al13Hn (n = 1–13) clusters. Population analysis has shown significant charge transfer occurring from the Al cluster to the H atoms. The population for Al13 varies from 0.24 (Al13H) to 2.83 (Al13H13). The shape of Al13 moieties in the Al13Hn (n ≥ 8) clusters is significantly distorted from the icosahedral structure of Al13 and is a “cagelike” form. Al13H13 has a capped icosahedron as the ground-state structure, similar to B13H13, while the shape of B13 (planar) is different from Al13 (icosahedral). The Al13H13 cluster is predicted to be exceptionally stable on the basis of the high stabilization energy and the negative nucleus independent chemical shift value.
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36.40.Qv Stability and fragmentation of clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
31.15.E- Density-functional theory
34.70.+e Charge transfer
33.25.+k Nuclear resonance and relaxation

An ab initio study on the ground and low-lying doublet electronic states of SbO2

Edmond P. F. Lee, John M. Dyke, Foo-Tim Chau, Wan-Ki Chow, and Daniel K. W. Mok

J. Chem. Phys. 125, 064307 (2006); http://dx.doi.org/10.1063/1.2335445 (12 pages) | Cited 2 times

Online Publication Date: 11 August 2006

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Geometry optimization and harmonic vibrational frequency calculations have been carried out on the low-lying doublet electronic states of antimony dioxide (SbO2) employing a variety of ab initio methods, including the complete active space self-consistent field/multireference configuration interaction and the RCCSD(T) methods. Both large and small core relativistic effective core potentials were used for Sb in these calculations, together with valence basis sets of up to aug-cc-pV5Z quality. Contributions from outer core correlation and off-diagonal spin-orbit interaction to relative electronic energies have been calculated. The ground electronic state of SbO2 is determined to be the math math state, as is the case for dioxides of other lighter group 15 p-block (or group VA) elements. However, the math math and math math states are estimated to be only 4.1 and 10.7 kcal/mole above the math math state, respectively, at the complete basis set limit. Reliable vertical excitation energies from the math math state to low-lying excited states of SbO2 have been computed with a view to assist future spectral assignments of the absorption and/or laser-induced fluorescence spectra of SbO2, when they become available.
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31.15.A- Ab initio calculations
31.15.V- Electron correlation calculations for atoms, ions and molecules
31.15.bw Coupled-cluster theory

Renner-Teller quantum dynamics of the N(math)+H2→NH+H reaction

Paolo Defazio and Carlo Petrongolo

J. Chem. Phys. 125, 064308 (2006); http://dx.doi.org/10.1063/1.2229212 (8 pages) | Cited 20 times

Online Publication Date: 11 August 2006

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We present the Born-Oppenheimer (BO) and Renner-Teller (RT) quantum dynamics of the reaction math(math)+math(math math)→NH(math math)+H(math), considering the NH2 electronic states math math and math math. These states correlate to the same math linear species, are coupled by RT nonadiabatic effects, and give NH(math math)+H and NH(math math)+H, respectively. We develop the Hamiltonian matrix elements in the R embedding of the Jacobi coordinates and in the adiabatic electronic representation, using the permutation-inversion symmetry, and taking into account the nuclear-spin statistics. Collision observables are calculated via the real wave-packet (WP) and flux methods, using the potential-energy surfaces of Santoro et al. [J. Phys. Chem. A 106, 8276 (2002)] . WP snapshots show that the reaction proceeds via an insertion mechanism, and that the RT-WP avoids the math math potential barrier, jumping from the excited to the ground surface and giving mainly the NH(math math) products. math math BO probabilities and cross sections show large tunnel effects and are approximately four to ten times larger than the math math ones. This implies a BO rate-constant ratio k(math math)/k(math math) ≈ 105 at 300 K, i.e., a negligible BO formation of NH(math math). When H2 is rotationally excited, RT couplings reduce slightly the math math reaction observables, but enhance strongly the math math reactivity. These couplings are important at all collision energies, reduce the collision threshold, and increase remarkably reaction probabilities and cross sections. The RT k(math math) is thus ∼ 3.3 order of magnitude larger than the BO value, and degeneracy-averaged, initial-state-resolved rate constants increase by ∼ 13% and by ∼ 47% at 300 and 500 K, respectively. Owing to an overestimation of the math math potential barrier, the calculated thermal rate is too low with respect to that observed, but we obtain a good agreement by shifting down the calculated cross section.
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34.50.Ez Rotational and vibrational energy transfer
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
31.50.Gh Surface crossings, non-adiabatic couplings

Non-Born-Oppenheimer variational calculation of the ground-state vibrational spectrum of LiH+

Sergiy Bubin and Ludwik Adamowicz

J. Chem. Phys. 125, 064309 (2006); http://dx.doi.org/10.1063/1.2244563 (4 pages) | Cited 6 times

Online Publication Date: 11 August 2006

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Very accurate, rigorous, variational, non-Born-Oppenheimer (non-BO) calculations have been performed for the fully symmetric, bound states of the LiH+ ion. These states correspond to the ground and excited vibrational states of LiH+ in the ground math electronic state. The non-BO wave functions of the states have been expanded in terms of spherical N-particle explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance and 5600 Gaussians were used for each state. The calculations that, to our knowledge, are the most accurate ever performed for a diatomic system with three electrons have yielded six bound states. Average interparticle distances and nucleus-nucleus correlation function plots are presented.
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33.20.Tp Vibrational analysis
31.15.xt Variational techniques

Automerization reaction of cyclobutadiene and its barrier height: An ab initio benchmark multireference average-quadratic coupled cluster study

Mirjana Eckert-Maksić, Mario Vazdar, Mario Barbatti, Hans Lischka, and Zvonimir B. Maksić

J. Chem. Phys. 125, 064310 (2006); http://dx.doi.org/10.1063/1.2222366 (9 pages) | Cited 23 times

Online Publication Date: 11 August 2006

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The problem of the double bond flipping interconversion of the two equivalent ground state structures of cyclobutadiene (CBD) is addressed at the multireference average-quadratic coupled cluster level of theory, which is capable of optimizing the structural parameters of the ground, transition, and excited states on an equal footing. The barrier height involving both the electronic and zero-point vibrational energy contributions is 6.3 kcal mol−1, which is higher than the best earlier theoretical estimate of 4.0 kcal mol−1. This result is confirmed by including into the reference space the orbitals of the CC σ bonds beyond the standard π orbital space. It places the present value into the middle of the range of the measured data (1.6–10 kcal mol−1). An adiabatic singlet-triplet energy gap of 7.4 kcal mol−1 between the transition state math and the first triplet math state is obtained. A low barrier height for the CBD automerization and a small ΔE(math,math) gap bear some relevance on the highly pronounced reactivity of CBD, which is briefly discussed.
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82.30.Qt Isomerization and rearrangement
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.15.A- Ab initio calculations
31.15.bw Coupled-cluster theory
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