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28 Oct 2007

Volume 127, Issue 16, Articles (16xxxx)

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Interference in Bohmian mechanics with complex action

Yair Goldfarb and David J. Tannor

J. Chem. Phys. 127, 161101 (2007); http://dx.doi.org/10.1063/1.2794029 (4 pages) | Cited 25 times

Online Publication Date: 25 October 2007

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In recent years, intensive effort has gone into developing numerical tools for exact quantum mechanical calculations that are based on Bohmian mechanics. As part of this effort we have recently developed as alternative formulation of Bohmian mechanics in which the quantum action S is taken to be complex [ Y. Goldfarb et al., J. Chem. Phys. 125, 231103 (2006) ]. In the alternative formulation there is a significant reduction in the magnitude of the quantum force as compared with the conventional Bohmian formulation, at the price of propagating complex trajectories. In this paper we show that Bohmian mechanics with complex action is able to overcome the main computational limitation of conventional Bohmian methods—the propagation of wave functions once nodes set in. In the vicinity of nodes, the quantum force in conventional Bohmian formulations exhibits rapid oscillations that present a severe numerical challenge. We show that within complex Bohmian mechanics, multiple complex initial conditions can lead to the same real final position, allowing for the accurate description of nodes as a sum of the contribution from two or more crossing trajectories. The idea is illustrated on the reflection amplitude from a one-dimensional Eckart barrier. We believe that trajectory crossing, although in contradiction to the conventional Bohmian trajectory interpretation, provides an important new tool for dealing with the nodal problem in Bohmian methods.
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02.20.Uw Quantum groups

Janus balance of amphiphilic colloidal particles

Shan Jiang and Steve Granick

J. Chem. Phys. 127, 161102 (2007); http://dx.doi.org/10.1063/1.2803420 (4 pages) | Cited 19 times

Online Publication Date: 29 October 2007

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We introduce the notion of “Janus balance” (J), defined as the dimensionless ratio of work to transfer an amphiphilic colloidal particle (a “Janus particle”) from the oil-water interface into the oil phase, normalized by the work needed to move it into the water phase. The J value can be calculated simply from the interfacial contact angle and the geometry of Janus particles, without the need to know the interfacial energy. It is demonstrated that Janus particles of the same chemical composition but different geometries will have the highest adsorption energy when J = 1. Even for particles of homogeneous chemical makeup, the Janus balance concept can be applied when considering the contact angle hysteresis in desorbing the particle from equilibrium into the water or oil phase. The Janus balance concept may enable predictions of how a Janus particle behaves with respect to efficiency and function as a solid surfactant, as the Janus balance of solid surfactants is the analog of the classical hydrophile-lipophile balance of small surfactant molecules.
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82.70.Dd Colloids
68.03.Cd Surface tension and related phenomena
68.43.Mn Adsorption kinetics
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back to top Theoretical Methods and Algorithms

Reaction coordinates and transition pathways of rare events via forward flux sampling

Ernesto E. Borrero and Fernando A. Escobedo

J. Chem. Phys. 127, 164101 (2007); http://dx.doi.org/10.1063/1.2776270 (17 pages) | Cited 20 times

Online Publication Date: 22 October 2007

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A new approach is developed for identifying suitable reaction coordinates to describe the progression of rare events in complex systems. The method is based on the forward flux sampling (FFS) technique and standard least-square estimation (LSE) and it is denoted as FFS-LSE. The FFS algorithm generates trajectories for the transition between stable states as chains of partially connected paths, which can then be used to obtain “on-the-fly” estimates for the committor probability to the final region, pB. These pB data are then used to screen a set of candidate collective properties for an optimal order parameter (i.e., reaction coordinate) that depends on a few relevant variables. LSE is used to find the coefficients of the proposed reaction coordinate model and an analysis of variance is used to determine the significant terms in the model. The method is demonstrated for several test systems, including the folding of a lattice protein. It is shown that a simple approximation to pB via a model linear on energy and number of native contacts is sufficient to describe the intrinsic dynamics of the protein system and to ensure an efficient sampling of pathways. In addition, since the pB surface found from the FFS-LSE approach leads to the identification of the transition state ensemble, mechanistic details of the dynamics of the system can be readily obtained during a single FFS-type simulation without the need to perform additional committor simulations.
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87.14.E- Proteins
87.15.Cc Folding: thermodynamics, statistical mechanics, models, and pathways
87.15.R- Reactions and kinetics
82.20.Bc State selected dynamics and product distribution

Radical hydrogen bonding: Origin of stability of radical-molecule complexes

Heriberto Hernández-Soto, Frank Weinhold, and Joseph S. Francisco

J. Chem. Phys. 127, 164102 (2007); http://dx.doi.org/10.1063/1.2784558 (10 pages) | Cited 10 times

Online Publication Date: 22 October 2007

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Natural bond orbital analysis is used to investigate the nature of hydrogen bonding in a series of binary open-shell complexes involving hydroperoxy radical HO2⋯X and analogous closed-shell H2O2⋯X complexes (where X = H2O, H2O2, HONO, HONO2, CH3OH, HCOOH, CH3COOH, and H2SO4) in order to elucidate and identify the electronic factors responsible for the strength of radical hydrogen bonds. Results from this study suggest that the radical species strongly alters the strength of the characteristic n→σ* donor-acceptor interaction in the hydrogen bonding. This interaction is found to contribute to the unusually strong binding in radical-molecule complexes. These findings have important new ramifications for our fundamental understanding of radical hydrogen bonds.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.30.Rs Hydrogen bonding, hydrophilic effects
33.15.Fm Bond strengths, dissociation energies

Frozen core and effective core potentials in symmetry-adapted perturbation theory

Konrad Patkowski and Krzysztof Szalewicz

J. Chem. Phys. 127, 164103 (2007); http://dx.doi.org/10.1063/1.2784391 (17 pages) | Cited 9 times

Online Publication Date: 22 October 2007

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The application of the frozen-core approximation (FCA) and effective core potentials (ECPs) within symmetry-adapted perturbation theory (SAPT) has been investigated and implemented. Unlike in the case of conventional electronic-structure theories, the development of a frozen-core version of SAPT is not straightforward. In particular, the FCA realizations neglecting excitations from core orbitals and restricting all summation indices to valence orbitals only are no longer equivalent. It is shown that it is necessary in SAPT to keep some terms containing products of the valence orbitals of one monomer and the core orbitals of the other one in the exchange-energy components. When these terms are included or, equivalently, the “infinite-excitation-energy” approximation omitting only the excitations from the core orbitals is used, the accuracy of the frozen-core approximation in SAPT matches that obtained in supermolecular perturbational and coupled-cluster methods. If these terms are neglected, i.e., within the “index-range-restriction” approximation, several exchange corrections are significantly underestimated. When ECPs are used in SAPT, the accuracy of the interaction energies is as good as in conventional supermolecular methods, provided that the residual supermolecular Hartree-Fock term is included. We have found that only some types of ECPs can be reliably used for calculations of interaction energies both in SAPT and in supermolecular approaches. For systems containing heavy atoms, both FCA and the use of ECPs lead to very significant savings of computer time.
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31.15.xp Perturbation theory
34.20.Gj Intermolecular and atom-molecule potentials and forces

Complex trajectories sans isochrones: Quantum barrier scattering with rectilinear constant velocity trajectories

Brad A. Rowland and Robert E. Wyatt

J. Chem. Phys. 127, 164104 (2007); http://dx.doi.org/10.1063/1.2790006 (7 pages) | Cited 13 times

Online Publication Date: 23 October 2007

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One of the major obstacles in employing complex-valued trajectory methods for quantum barrier scattering calculations is the search for isochrones. In this study, complex-valued derivative propagation method trajectories in the arbitrary Lagrangian-Eulerian frame are employed to solve the complex Hamilton-Jacobi equation for quantum barrier scattering problems employing constant velocity trajectories moving along rectilinear paths whose initial points can be in the complex plane or even along the real axis. It is shown that this effectively removes the need for isochrones for barrier transmission problems. Model problems tested include the Eckart, Gaussian, and metastable quadratic+cubic potentials over a variety of wave packet energies. For comparison, the “exact” solution is computed from the time-dependent Schrödinger equation via pseudospectral methods.
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82.20.Fd Collision theories; trajectory models
03.65.Nk Scattering theory
03.65.Ge Solutions of wave equations: bound states

Linear response coupled cluster singles and doubles approach with modified spectral resolution of the similarity transformed Hamiltonian

Karol Kowalski, Jeff R. Hammond, and Wibe A. de Jong

J. Chem. Phys. 127, 164105 (2007); http://dx.doi.org/10.1063/1.2795708 (9 pages) | Cited 4 times

Online Publication Date: 23 October 2007

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This paper discusses practical scheme for correcting the linear response coupled cluster with singles and doubles (CCSD) equations by shifting the poles corresponding to the equation-of-motion CCSD excitation energies by adding noniterative corrections due to triples. A simple criterion is derived for the excited states to be corrected in the spectral resolution of similarity transformed Hamiltonian on the CCSD level. Benchmark calculations were performed to compare the accuracies of static and dynamic polarizabilities obtained in this way with the CC3 and CCSDT counterparts.
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31.15.bw Coupled-cluster theory
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
32.30.-r Atomic spectra

Ultrasoft pseudopotentials in time-dependent density-functional theory

Brent Walker and Ralph Gebauer

J. Chem. Phys. 127, 164106 (2007); http://dx.doi.org/10.1063/1.2786999 (9 pages) | Cited 12 times

Online Publication Date: 23 October 2007

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We describe an efficient formulation allowing the use of ultrasoft pseudopotentials (USPPs) in plane wave based time-dependent density-functional theory. The practical steps required to implement USPP functionality within real time propagation schemes and linear-response schemes based on Lanczos algorithms are provided. The functioning of the methodology is demonstrated by calculations of the optical absorption spectra of the fullerene C60, using both real time propagation and the Lanczos/linear-response approaches. Comparisons between the rates of convergence of the optical spectra with the number of applications of the Hamiltonian required in calculations with ultrasoft pseudopotentials and norm-conserving pseudopotentials show clearly the benefits provided by the use of USPP.
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33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.15.E- Density-functional theory

Sequential quadratic programming method for determining the minimum energy path

Steven K. Burger and Weitao Yang

J. Chem. Phys. 127, 164107 (2007); http://dx.doi.org/10.1063/1.2780147 (7 pages) | Cited 9 times

Online Publication Date: 23 October 2007

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A new method, referred to as the sequential quadratic programming method, is presented for determining minimum energy paths. The method is based on minimizing the points representing the path in the subspace perpendicular to the tangent of the path while using a penalty term to prevent kinks from forming. Rather than taking one full step, the minimization is divided into a number of sequential steps on an approximate quadratic surface. The resulting method can efficiently determine the reaction mechanism, from which transition state can be easily identified and refined with other methods. To improve the resolution of the path close to the transition state, points are clustered close to this region with a reparametrization scheme. The usefulness of the algorithm is demonstrated for the Müller-Brown potential, amide hydrolysis, and an 89 atom cluster taken from the active site of 4-oxalocrotonate tautomerase for the reaction which catalyzes 2-oxo-4-hexenedioate to the intermediate 2-hydroxy-2,4-hexadienedioate.
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82.20.Db Transition state theory and statistical theories of rate constants
82.20.Hf Product distribution
82.20.Kh Potential energy surfaces for chemical reactions

Cartesian formulation of the mobile block Hessian approach to vibrational analysis in partially optimized systems

A. Ghysels, D. Van Neck, and M. Waroquier

J. Chem. Phys. 127, 164108 (2007); http://dx.doi.org/10.1063/1.2789429 (9 pages) | Cited 7 times

Online Publication Date: 23 October 2007

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Partial optimization is a useful technique to reduce the computational load in simulations of extended systems. In such nonequilibrium structures, the accurate calculation of localized vibrational modes can be troublesome, since the standard normal mode analysis becomes inappropriate. In a previous paper [ A. Ghysels et al., J. Chem. Phys. 126, 224102 (2007) ], the mobile block Hessian (MBH) approach was presented to deal with the vibrational analysis in partially optimized systems. In the MBH model, the nonoptimized regions of the system are represented by one or several blocks, which can move as rigid bodies with respect to the atoms of the optimized region. In this way unphysical imaginary frequencies are avoided and the translational/rotational invariance of the potential energy surface is fully respected. In this paper we focus on issues concerning the practical numerical implementation of the MBH model. The MBH normal mode equations are worked out for several coordinate choices. The introduction of a consistent group-theoretical notation facilitates the treatment of both the case of a single block and the case of multiple blocks. Special attention is paid to the formulation in terms of Cartesian variables, in order to provide a link with the standard output of common molecular modeling programs.
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33.20.Tp Vibrational analysis
31.50.-x Potential energy surfaces
31.15.-p Calculations and mathematical techniques in atomic and molecular physics

Accurate extrapolation of electron correlation energies from small basis sets

Dirk Bakowies

J. Chem. Phys. 127, 164109 (2007); http://dx.doi.org/10.1063/1.2768359 (12 pages) | Cited 8 times

Online Publication Date: 25 October 2007

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A new two-point scheme is proposed for the extrapolation of electron correlation energies obtained with small basis sets. Using the series of correlation-consistent polarized valence basis sets, cc-pVXZ, the basis set truncation error is expressed as δEX∝(X+ξi)−γ. The angular momentum offset ξi captures differences in effective rates of convergence previously observed for first-row molecules. It is based on simple electron counts and tends to values close to 0 for hydrogen-rich compounds and values closer to 1 for pure first-row compounds containing several electronegative atoms. The formula is motivated theoretically by the structure of correlation-consistent basis sets which include basis functions up to angular momentum L = X−1 for hydrogen and helium and up to L = X for first-row atoms. It contains three parameters which are calibrated against a large set of 105 reference molecules (H, C, N, O, F) for extrapolations of MP2 and CCSD valence-shell correlation energies from double- and triple-zeta (DT) and triple- and quadruple-zeta (TQ) basis sets. The new model is shown to be three to five times more accurate than previous two-point schemes using a single parameter, and (TQ) extrapolations are found to reproduce a small set of available R12 reference data better than even (56) extrapolations using the conventional asymptotic limit formula δEX∝X−3. Applications to a small selection of boron compounds and to neon show very satisfactory results as well. Limitations of the model are discussed.
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31.15.V- Electron correlation calculations for atoms, ions and molecules
31.15.xp Perturbation theory
31.15.bw Coupled-cluster theory

Benchmark full configuration interaction and equation-of-motion coupled-cluster model with single and double substitutions for ionized systems results for prototypical charge transfer systems: Noncovalent ionized dimers

Piotr A. Pieniazek, Stephen A. Arnstein, Stephen E. Bradforth, Anna I. Krylov, and C. David Sherrill

J. Chem. Phys. 127, 164110 (2007); http://dx.doi.org/10.1063/1.2795709 (19 pages) | Cited 24 times

Online Publication Date: 26 October 2007

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Benchmark full configuration interaction and equation-of-motion coupled-cluster model with single and double substitutions for ionized systems (EOM-IP-CCSD) results are presented for prototypical charge transfer species. EOM-IP-CCSD describes these doublet systems based on the closed-shell reference and thus avoids the doublet instability problem. The studied quantities are associated with the quality of the potential energy surface (PES) along the charge transfer coordinate and distribution of the charge between fragments. It is found that EOM-IP-CCSD is capable of describing accurately both the charge-localized and charge-delocalized systems, yielding accurate charge distributions and energies. This is in stark contrast with the methods based on the open-shell reference, which overlocalize the charge and produce a PES cusp when the fragments are indistinguishable.
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31.15.V- Electron correlation calculations for atoms, ions and molecules
31.15.bw Coupled-cluster theory
31.50.-x Potential energy surfaces

Troubleshooting time-dependent density-functional theory for photochemical applications: Oxirane

Felipe Cordova, L. Joubert Doriol, Andrei Ipatov, Mark E. Casida, Claudia Filippi, and Alberto Vela

J. Chem. Phys. 127, 164111 (2007); http://dx.doi.org/10.1063/1.2786997 (18 pages) | Cited 19 times

Online Publication Date: 29 October 2007

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The development of analytic-gradient methodology for excited states within conventional time-dependent density-functional theory (TDDFT) would seem to offer a relatively inexpensive alternative to better established quantum-chemical approaches for the modeling of photochemical reactions. However, even though TDDFT is formally exact, practical calculations involve the use of approximate functional, in particular the TDDFT adiabatic approximation, the use of which in photochemical applications must be further validated. Here, we investigate the prototypical case of the symmetric CC ring opening of oxirane. We demonstrate by direct comparison with the results of high-quality quantum Monte Carlo calculations that, far from being an approximation on TDDFT, the Tamm-Dancoff approximation is a practical necessity for avoiding triplet instabilities and singlet near instabilities, thus helping maintain energetically reasonable excited-state potential energy surfaces during bond breaking. Other difficulties one would encounter in modeling oxirane photodynamics are pointed out.
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82.50.-m Photochemistry
31.15.E- Density-functional theory
31.50.Df Potential energy surfaces for excited electronic states
31.50.Gh Surface crossings, non-adiabatic couplings

Calculation of the zero-field splitting tensor on the basis of hybrid density functional and Hartree-Fock theory

Frank Neese

J. Chem. Phys. 127, 164112 (2007); http://dx.doi.org/10.1063/1.2772857 (9 pages) | Cited 51 times

Online Publication Date: 30 October 2007

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The zero-field splitting (ZFS) (expressed in terms of the D tensor) is the leading spin-Hamiltonian parameter for systems with a ground state spin S>1/2. To first order in perturbation theory, the ZFS arises from the direct spin-spin dipole-dipole interaction. To second order, contributions arise from spin-orbit coupling (SOC). The latter contributions are difficult to treat since the SOC mixes states of different multiplicities. This is an aspect of dominant importance for the correct prediction of the D tensor. In this work, the theory of the D tensor is discussed from the point of view of analytic derivative theory. Starting from a general earlier perturbation treatment [ F. Neese and E. I. Soloman, Inorg. Chem. 37, 6568 (1998) ], straightforward response equations are derived that are readily transferred to the self-consistent field (SCF) Hartree-Fock (HF) or density functional theory (DFT) framework. The main additional effort in such calculations arises from the solution of nine sets of nonstandard coupled-perturbed SCF equations. These equations have been implemented together with the spin-orbit mean-field representation of the SOC operator and a mean-field treatment of the direct spin-spin interaction into the ORCA electronic structure program. A series of test calculations on diatomic molecules with accurately known zero-field splittings shows that the new approach corrects most of the shortcomings of previous DFT based methods and, on average, leads to predictions within 10% of the experimental values. The slope of the correlation line is essentially unity for the B3LYP and BLYP functionals compared to ∼ 0.5 in previous treatments.
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31.30.Gs Hyperfine interactions and isotope effects
33.15.Pw Fine and hyperfine structure
31.15.E- Density-functional theory
31.15.xp Perturbation theory
31.15.xr Self-consistent-field methods
33.25.+k Nuclear resonance and relaxation

Efficient density-functional theory integrations by locally augmented radial grids

Jürgen Gräfenstein and Dieter Cremer

J. Chem. Phys. 127, 164113 (2007); http://dx.doi.org/10.1063/1.2794038 (7 pages) | Cited 10 times

Online Publication Date: 30 October 2007

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Standard density-functional theory integration grids have proven insufficient for the meta generalized gradient approximation description of weakly bound complexes [ E. R. Johnson et al., Chem. Phys. Lett. 394, 334 (2004) ]. This is caused by an insufficient radial resolution in the valence region of frequently used standard grids. We present an algorithm for the construction of locally augmented radial grids, which allows us to enhance the resolution of a given radial grid in a specified region, thus increasing the accuracy of the standard grid in a cost-efficient way. Test calculations with the Van Voorhis-Scuseria exchange and correlation functional for the Ar dimer confirm that a suitably constructed, locally augmented radial grid with 100 points provides an accuracy competitive to that of a 250-point nonaugmented grid. Time savings and possible applications for locally augmented radial grids are discussed.
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31.15.E- Density-functional theory

Projection formalism for constrained dynamical systems: From Newtonian to Hamiltonian mechanics

Gerald R. Kneller

J. Chem. Phys. 127, 164114 (2007); http://dx.doi.org/10.1063/1.2779326 (5 pages) | Cited 4 times

Online Publication Date: 30 October 2007

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The Hamiltonian of a holonomically constrained dynamical many-particle system in Cartesian coordinates has been recently derived for applications in statistical mechanics [ G. R. Kneller, J. Chem. Phys. 125, 114107 (2006) ]. Using the same projector formalism, we show here the equivalence of the corresponding equations of motion with those obtained from a Newtonian and a Lagrangian description. In the case of Newtonian mechanics, the general case of nonholonomic constraints is considered, too.
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45.50.Jf Few- and many-body systems
05.20.-y Classical statistical mechanics

A comparison of two methods for selecting vibrational configuration interaction spaces on a heptatomic system: Ethylene oxide

Didier Bégué, Neil Gohaud, Claude Pouchan, Patrick Cassam-Chenaï, and Jacques Liévin

J. Chem. Phys. 127, 164115 (2007); http://dx.doi.org/10.1063/1.2795711 (10 pages) | Cited 34 times

Online Publication Date: 31 October 2007

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Two recently developed methods for solving the molecular vibrational Schrödinger equation, namely, the parallel vibrational multiple window configuration interaction and the vibrational mean field configuration interaction, are presented and compared on the same potential energy surface of ethylene oxide, c-C2H4O. It is demonstrated on this heptatomic system with strong resonances that both approaches converge towards the same fundamental frequencies. This confirms their ability to tackle the vibrational problem of large molecules for which full configuration interaction calculations are not tractable.
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31.15.vq Electron correlation calculations for polyatomic molecules
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.50.Bc Potential energy surfaces for ground electronic states

An improved replica-exchange sampling method: Temperature intervals with global energy reassignment

Xianfeng Li, Christopher P. O’Brien, Galen Collier, Nadeem A. Vellore, Feng Wang, Robert A. Latour, David A. Bruce, and Steven J. Stuart

J. Chem. Phys. 127, 164116 (2007); http://dx.doi.org/10.1063/1.2780152 (10 pages) | Cited 8 times

Online Publication Date: 31 October 2007

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In a molecular dynamics (MD) simulation, representative sampling over the entire phase space is desired to obtain an accurate canonical distribution at a given temperature. For large molecules, such as proteins, this is problematic because systems tend to become trapped in local energy minima. The extensively used replica-exchange molecular dynamics (REMD) simulation technique overcomes this kinetic-trapping problem by allowing Boltzmann-weighted configuration exchange processes to occur between numerous thermally adjacent and compositionally identical simulations that are thermostated at sequentially higher temperatures. While the REMD method provides much better sampling than conventional MD, there are two substantial difficulties that are inherent in its application: (1) the large number of replicas that must be used to span a designated temperature range and (2) the subsequent long time required for configurations sampled at high temperatures to exchange down for potential inclusion within the low-temperature ensemble of interest. In this work, a new method based on temperature intervals with global energy reassignment (TIGER) is presented that overcomes both of these problems. A TIGER simulation is conducted as a series of short heating-sampling-quenching cycles. At the end of each cycle, the potential energies of all replicas are simultaneously compared at the same temperature using a Metropolis sampling method and then globally reassigned to the designated temperature levels. TIGER is compared with regular MD and REMD methods for the alanine dipeptide in water. The results indicate that TIGER increases sampling efficiency while substantially reducing the number of central processing units required for a comparable conventional REMD simulation.
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87.14.E- Proteins
87.15.B- Structure of biomolecules
36.20.-r Macromolecules and polymer molecules
31.15.xv Molecular dynamics and other numerical methods

Local hybrid functionals based on density matrix products

Benjamin G. Janesko and Gustavo E. Scuseria

J. Chem. Phys. 127, 164117 (2007); http://dx.doi.org/10.1063/1.2784406 (11 pages) | Cited 16 times

Online Publication Date: 31 October 2007

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We present a novel similarity metric comparing exact and semilocal density functional theory (DFT) exchange holes in real space. This metric is obtained from the product of the one-particle density matrix and the uniform electron gas model density matrix. The metric is bound between 0 and 1, 1 in the uniform electron gas, 0 in regions asymptotically far from finite systems, and can detect delocalization of the exact exchange hole and effective fractional occupations. We also present a parameter-free local hybrid functional that uses this similarity metric to locally mix exact and semilocal DFT exchange energy densities. The resulting functional gives better thermochemistry and reaction barrier heights than our original local hybrids [ Jaramillo et al., J. Chem. Phys. 118, 1068 (2003) ], while retaining moderate accuracy for symmetric radical cation dimers.
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31.15.E- Density-functional theory
34.70.+e Charge transfer
31.15.V- Electron correlation calculations for atoms, ions and molecules

Calculation of vibrational frequencies through a variational reduced-coupling approach

Yohann Scribano and David M. Benoit

J. Chem. Phys. 127, 164118 (2007); http://dx.doi.org/10.1063/1.2798104 (8 pages) | Cited 12 times

Online Publication Date: 31 October 2007

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In this study, we present a new method to perform accurate and efficient vibrational configuration interaction computations for large molecular systems. We use the vibrational self-consistent field (VSCF) method to compute an initial description of the vibrational wave function of the system, combined with the single-to-all approach to compute a sparse potential energy surface at the chosen ab initio level of theory. A Davidson scheme is then used to diagonalize the Hamiltonian matrix built on the VSCF virtual basis. Our method is applied to the computation of the OH-stretch frequency of formic acid and benzoic acid to demonstrate the efficiency and accuracy of this new technique.
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33.15.Mt Rotation, vibration, and vibration-rotation constants
36.20.Ng Vibrational and rotational structure, infrared and Raman spectra
31.15.xt Variational techniques
31.15.V- Electron correlation calculations for atoms, ions and molecules
31.50.-x Potential energy surfaces
31.15.xr Self-consistent-field methods

Configuration interaction based on constrained density functional theory: A multireference method

Qin Wu, Chiao-Lun Cheng, and Troy Van Voorhis

J. Chem. Phys. 127, 164119 (2007); http://dx.doi.org/10.1063/1.2800022 (9 pages) | Cited 25 times

Online Publication Date: 31 October 2007

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Existing density functional theory (DFT) methods are typically very effective in capturing dynamic correlation, but run into difficulty treating near-degenerate systems where static correlation becomes important. In this work, we propose a configuration interaction (CI) method that allows one to use a multireference approach to treat static correlation but incorporates DFT’s efficacy for the dynamic part as well. The new technique uses localized charge or spin states built by a constrained DFT approach to construct an active space in which the effective Hamiltonian matrix is built. These local configurations have significantly less static correlation compared to their delocalized counterparts and possess an essentially constant amount of self-interaction error. Thus their energies can be reliably calculated by DFT with existing functionals. Using a small number of local configurations as different references in the active space, a simple CI step is then able to recover the static correlation missing from the localized states. Practical issues of choosing configurations and adjusting constraint values are discussed, employing as examples the ground state dissociation curves of H2+, H2, and LiF. Excellent results are obtained for these curves at all interatomic distances, which is a strong indication that this method can be used to accurately describe bond breaking and forming processes.
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31.15.V- Electron correlation calculations for atoms, ions and molecules
31.15.E- Density-functional theory
33.15.Fm Bond strengths, dissociation energies
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Excitation, dynamics, and control of rotationally autoionizing Rydberg states of H2

A. Kirrander, H. H. Fielding, and Ch. Jungen

J. Chem. Phys. 127, 164301 (2007); http://dx.doi.org/10.1063/1.2798764 (10 pages) | Cited 9 times

Online Publication Date: 22 October 2007

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The dynamics of rotationally autoionizing Rydberg states of molecular hydrogen is investigated using a time-dependent extension of multichannel quantum defect theory, in which the time-dependent wave packets are constructed using first-order perturbation theory. An analytical expression for the complex excitation function for a sequence of Gaussian excitation pulses is derived and then employed to investigate the influence of pairs of pulses with well-defined phase differences on the decay dynamics and final-state composition.
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31.15.vj Electron correlation calculations for atoms and ions: excited states
31.15.xp Perturbation theory
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Eh Autoionization, photoionization, and photodetachment

Rotational spectra of rare isotopic species of bromofluoromethane: Determination of the equilibrium structure from ab initio calculations and microwave spectroscopy

Cristina Puzzarini, Gabriele Cazzoli, Agostino Baldacci, Alessandro Baldan, Christine Michauk, and Jürgen Gauss

J. Chem. Phys. 127, 164302 (2007); http://dx.doi.org/10.1063/1.2790895 (10 pages) | Cited 7 times

Online Publication Date: 23 October 2007

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Guided by theoretical predictions, the rotational spectra of the mono- and bideuterated species of bromofluoromethane, CDH79BrF, CDH81BrF, CD279BrF, and CD281BrF, have been recorded for the first time. Assignment of a few hundred rotational transitions led to the accurate determination of the ground-state rotational constants, all of the quartic and most of the sextic centrifugal distortion constants, as well as the full bromine quadrupole-coupling tensor for both 79Br and 81Br, in good agreement with corresponding theoretical predictions based on high-level coupled-cluster calculations. The rotational spectra of the 13C containing species 13CH279BrF and 13CH281BrF have been observed in natural abundance and have been assigned, thus allowing the determination of the rotational and centrifugal distortion constants as well as the bromine quadrupole-coupling tensor. Furthermore, empirical equilibrium structures have been obtained within a least-squares fit procedure using the available experimental ground-state rotational constants for various isotopic species. Vibrational effects have been accounted for in the analysis using vibration-rotation interaction constants derived from anharmonic force fields computed at the second-order Møller-Plesset perturbation theory as well as coupled-cluster (CC) levels. The empirical equilibrium geometries obtained in this way agree well with the corresponding theoretical predictions obtained from CC calculations [at the CCSD(T) level] after extrapolation to the complete basis set limit and inclusion of core-valence correlation corrections and relativistic effects.
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33.20.Bx Radio-frequency and microwave spectra
31.15.bw Coupled-cluster theory
33.20.Sn Rotational analysis
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.A- Ab initio calculations
33.15.Mt Rotation, vibration, and vibration-rotation constants

Theoretical study of the electronic structure of HXY/XYH radicals (X=C,Si;Y=O,S)

Ignacio Pérez-Juste and Luis Carballeira

J. Chem. Phys. 127, 164303 (2007); http://dx.doi.org/10.1063/1.2777138 (11 pages) | Cited 3 times

Online Publication Date: 24 October 2007

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The electronic structures of the HXY/XYH compounds (X=C,Si;Y=O,S) on the 2A′ electronic ground state were investigated by applying the natural bond orbital (NBO) method to the computed B3LYP/6-311G** wave functions. Different localized structures are proposed for the HXY and XYH isomers and the central XY unit is described as intermediate between a double and a triple bond in HCO, HCS, HSiO, and HSiS, similar to a double bond in COH, CSH, and SiSH, and clearly a single bond in SiOH. Through the comparison between the NBO results for the diatomic and hydrogenated compounds, the energy preferences on each pair of isomers and the computed geometrical parameters are explained. According to the structures proposed, the HXY compounds are σ radicals with the spin density distributed along the molecular framework, while the XYH compounds are π radicals with most of the unpaired spin located on an almost pure p orbital of the X atom. Finally, the amounts of spin density on natural atomic orbitals provided by the NBO method are used to explain the computed values of the isotropic and anisotropic hyperfine coupling constants.
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31.15.E- Density-functional theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Dj Interatomic distances and angles
33.15.Fm Bond strengths, dissociation energies
33.15.Pw Fine and hyperfine structure

Hg+Br→HgBr recombination and collision-induced dissociation dynamics

Benjamin C. Shepler, Nikolai B. Balabanov, and Kirk A. Peterson

J. Chem. Phys. 127, 164304 (2007); http://dx.doi.org/10.1063/1.2777142 (10 pages) | Cited 9 times

Online Publication Date: 24 October 2007

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A global potential energy surface has been constructed for the system HgBr+Ar→Hg+Br+Ar to determine temperature dependent rate constants for the collision-induced dissociation (CID) and recombination of Hg and Br atoms. The surface was decomposed using a many-body expansion. Accurate two-body potentials for HgBr, HgAr, and ArBr were calculated using coupled cluster theory with single and double excitations and a perturbative treatment of triple excitations [CCSD(T)], as well as the multireference averaged coupled pair functional method. Correlation consistent basis sets were used to extrapolate to the complete basis set limit and corrections were included to account for scalar and spin-orbit relativistic effects, core-valence correlation, and the Lamb shift. The three-body potential was computed with the CCSD(T) method and triple-zeta quality basis sets. Quasiclassical trajectories using the final analytical potential surface were directly carried out on the CID of HgBr by Ar for a large sampling of initial rotational, vibrational, and collision energies. The recombination rate of Hg and Br atoms is a likely first step in mercury depletion events that have been observed in the Arctic troposphere during polar sunrise. The effective second order rate constant for this process was determined in this work from the calculated CID rate as a function of temperature using the principle of detailed balance, which resulted in k(T) = 1.2×10−12 cm3 molecule−1 s−1 at 260 K and 1 bar pressure.
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82.33.Tb Atmospheric chemistry
82.30.Nr Association, addition, insertion, cluster formation
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies
31.15.bw Coupled-cluster theory
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