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21 Jun 2006

Volume 124, Issue 23, Articles (23xxxx)

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Metastable extension of the sublimation curve and the critical contact point

V. G. Baidakov and S. P. Protsenko

J. Chem. Phys. 124, 231101 (2006); http://dx.doi.org/10.1063/1.2183770 (3 pages) | Cited 2 times

Online Publication Date: 15 June 2006

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Molecular dynamics methods have been employed in order to calculate the (p,ρ,T)-properties and the internal energy of gas and crystal phases in stable and metastable equilibrium coexistence states for a model system consisting of 2048 Lennard-Jones particles. Thermal and caloric equations of state and the spinodal curves of the vapor and crystal phases have been determined. A new algorithm for the computation of phase equilibrium curves is suggested. Employing this method, the sublimation curve and its metastable extension to temperatures above the triple point have been calculated. It is found that the crystal-gas phase equilibrium terminates on the spinodal of the superheated crystal. The point of contact of the sublimation line and the spinodal is a singular point of the thermodynamic surface of states of a simple system considered.
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64.70.Hz Solid-vapor transitions
64.60.F- Equilibrium properties near critical points, critical exponents
64.60.My Metastable phases
64.10.+h General theory of equations of state and phase equilibria

Non-meanfield deterministic limits in chemical reaction kinetics

R. E. Lee DeVille, Cyrill B. Muratov, and Eric Vanden-Eijnden

J. Chem. Phys. 124, 231102 (2006); http://dx.doi.org/10.1063/1.2217013 (4 pages) | Cited 9 times

Online Publication Date: 20 June 2006

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A general mechanism is proposed by which small intrinsic fluctuations in a system far from equilibrium can result in nearly deterministic dynamical behaviors which are markedly distinct from those realized in the meanfield limit. The mechanism is demonstrated for the kinetic Monte Carlo version of the Schnakenberg reaction where we identified a scaling limit in which the global deterministic bifurcation picture is fundamentally altered by fluctuations. Numerical simulations of the model are found to be in quantitative agreement with theoretical predictions.
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82.20.Db Transition state theory and statistical theories of rate constants
82.30.-b Specific chemical reactions; reaction mechanisms
82.40.Bj Oscillations, chaos, and bifurcations

High coordination chemically bound compounds of noble gases with hydrocarbons: Ng(CCH)4 and Ng(CCH)6, (Ng = Xe or Kr)

Li Sheng and R. Benny Gerber

J. Chem. Phys. 124, 231103 (2006); http://dx.doi.org/10.1063/1.2217734 (3 pages) | Cited 3 times

Online Publication Date: 20 June 2006

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Ab initio calculations predict the existence of the compounds Ng(CCH)4 and Ng(CCH)6, where Ng = Xe or Kr. Presently known organic noble gas compounds have a coordination number of two at most. The Ng(CCH)4 molecules have D4h symmetry, and Ng(CCH)6 molecules have Oh symmetry. The bonding in all these compounds is partly ionic and partly covalent, with significant contributions from both types of bonding. The relatively high vibrational frequencies and the substantial Ng-(CCH) binding energy in these species indicate that these compounds should be fairly stable, at least in cryogenic conditions. These compounds could be a very interesting addition to the range of known organic noble gas compounds. Suggestions are made on possible approaches to their preparation.
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31.15.A- Ab initio calculations
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Fm Bond strengths, dissociation energies
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
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back to top Theoretical Methods and Algorithms

Quantum mechanical single molecule partition function from path integral Monte Carlo simulations

Shaji Chempath, Cristian Predescu, and Alexis T. Bell

J. Chem. Phys. 124, 234101 (2006); http://dx.doi.org/10.1063/1.2196885 (12 pages) | Cited 5 times

Online Publication Date: 15 June 2006

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An algorithm for calculating the partition function of a molecule with the path integral Monte Carlo method is presented. Staged thermodynamic perturbation with respect to a reference harmonic potential is utilized to evaluate the ratio of partition functions. Parallel tempering and a new Monte Carlo estimator for the ratio of partition functions are implemented here to achieve well converged simulations that give an accuracy of 0.04 kcal/mol in the reported free energies. The method is applied to various test systems, including a catalytic system composed of 18 atoms. Absolute free energies calculated by this method lead to corrections as large as 2.6 kcal/mol at 300 K for some of the examples presented.
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82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
82.60.Cx Enthalpies of combustion, reaction, and formation
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Ej Quantum theory of reaction cross section

Iterative fluctuation charge model: A new variable charge molecular dynamics method

Ying Ma and Stephen H. Garofalini

J. Chem. Phys. 124, 234102 (2006); http://dx.doi.org/10.1063/1.2206578 (7 pages) | Cited 4 times

Online Publication Date: 15 June 2006

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In molecular simulations, calculation of environmentally dependent atomic charges is still a demanding task. Empirical and semiempirical methods have been proposed and applied to a wide range of problems with different success. In this paper, a new scheme based on the concept of electronegativity equalization is presented and its advantages over several other methods are discussed. This method is an extension of the fluctuation charge model [ S. W. Rick, S. J. Stuart, and B. J. Berne, J. Chem. Phys. 101, 6141 (1994) ]. By allowing multiple electronic iterations at each nuclear step, the condition of electronegativity equalization can be satisfied to a selected precision. Molecular dynamics simulations using this new method, as well as several other methods, are performed on α quartz. Analysis of the simulated results shows that it is advantageous to use the iterative fluctuation charge model in several different situations.
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31.15.xv Molecular dynamics and other numerical methods

Modeling the influence of a laser pulse on the potential energy surface in optimal molecular control theory

John D. Farnum, Gergely Gidofalvi, and David A. Mazziotti

J. Chem. Phys. 124, 234103 (2006); http://dx.doi.org/10.1063/1.2206585 (5 pages) | Cited 1 time

Online Publication Date: 16 June 2006

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Understanding and modeling the interaction between light and matter is essential to the theory of optical molecular control. While the effect of the electric field on a molecule’s electronic structure is often not included in control theory, it can be modeled in an optimal control algorithm by a set or toolkit of potential energy surfaces indexed by discrete values of the electric field strength where the surfaces are generated by Born-Oppenheimer electronic structure calculations that directly include the electric field. Using a new optimal control algorithm with a trigonometric mapping to limit the maximum field strength explicitly, we apply the surface-toolkit method to control the hydrogen fluoride molecule. Potential energy surfaces in the presence and absence of the electric field are created with two-electron reduced-density-matrix techniques. The population dynamics show that adjusting for changes in the electronic structure of the molecule beyond the static dipole approximation can be significant for designing a field that drives a realistic quantum system to its target observable.
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33.80.-b Photon interactions with molecules
31.50.-x Potential energy surfaces

Is the Ewald summation still necessary? Pairwise alternatives to the accepted standard for long-range electrostatics

Christopher J. Fennell and J. Daniel Gezelter

J. Chem. Phys. 124, 234104 (2006); http://dx.doi.org/10.1063/1.2206581 (12 pages) | Cited 56 times

Online Publication Date: 19 June 2006

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We investigate pairwise electrostatic interaction methods and show that there are viable computationally efficient (O(N)) alternatives to the Ewald summation for typical modern molecular simulations. These methods are extended from the damped and cutoff-neutralized Coulombic sum originally proposed by Wolf et al. [J. Chem. Phys. 110, 8255 (1999) ]. One of these, the damped shifted force method, shows a remarkable ability to reproduce the energetic and dynamic characteristics exhibited by simulations employing lattice summation techniques. Comparisons were performed with this and other pairwise methods against the smooth particle-mesh Ewald summation to see how well they reproduce the energetics and dynamics of a variety of molecular simulations.
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61.20.Ja Computer simulation of liquid structure

Symplectic splitting operator methods for the time-dependent Schrödinger equation

Sergio Blanes, Fernando Casas, and Ander Murua

J. Chem. Phys. 124, 234105 (2006); http://dx.doi.org/10.1063/1.2203609 (10 pages) | Cited 5 times

Online Publication Date: 20 June 2006

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We present a family of symplectic splitting methods especially tailored to solve numerically the time-dependent Schrödinger equation. When discretized in time, this equation can be recast in the form of a classical Hamiltonian system with a Hamiltonian function corresponding to a generalized high-dimensional separable harmonic oscillator. The structure of the system allows us to build highly efficient symplectic integrators at any order. The new methods are accurate, easy to implement, and very stable in comparison with other standard symplectic integrators.
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03.65.Ge Solutions of wave equations: bound states
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
03.65.Sq Semiclassical theories and applications

Analysis of the statistical error in umbrella sampling simulations by umbrella integration

Johannes Kästner and Walter Thiel

J. Chem. Phys. 124, 234106 (2006); http://dx.doi.org/10.1063/1.2206775 (7 pages) | Cited 15 times

Online Publication Date: 20 June 2006

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Umbrella sampling simulations, or biased molecular dynamics, can be used to calculate the free-energy change of a chemical reaction. We investigate the sources of different sampling errors and derive approximate expressions for the statistical errors when using harmonic restraints and umbrella integration analysis. This leads to generally applicable rules for the choice of the bias potential and the sampling parameters. Numerical results for simulations on an analytical model potential are presented for validation. While the derivations are based on umbrella integration analysis, the final error estimate is evaluated from the raw simulation data, and it may therefore be generally applicable as indicated by tests using the weighted histogram analysis method.
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82.20.Db Transition state theory and statistical theories of rate constants
82.30.-b Specific chemical reactions; reaction mechanisms
82.60.-s Chemical thermodynamics

Application of Gaussian-type geminals in local second-order Møller-Plesset perturbation theory

Robert Polly, Hans-Joachim Werner, Pål Dahle, and Peter R. Taylor

J. Chem. Phys. 124, 234107 (2006); http://dx.doi.org/10.1063/1.2202102 (11 pages) | Cited 20 times

Online Publication Date: 20 June 2006

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In this work Gaussian-type Geminals (GTGs) are applied in local second-order Møller-Plesset perturbation theory to improve the basis set convergence. Our implementation is based on the weak orthogonality functional of Szalewicz et al., [Chem. Phys. Lett. 91, 169 (1982); J. Chem. Phys. 78, 1420 (1983) ] and a newly developed program for calculating the necessary many-electron integrals. The local approximations together with GTGs in the treatment of the correlation energy are introduced and tested. First results for correlation energies of H2O, CH4, CO, C2H2, C2H4, H2CO, and N2H4 as well as some reaction and activation energies are presented. More than 97% of the valence-shell correlation energy is recovered using aug-cc-pVDZ basis sets and six GTGs per electron pair. The results are compared with conventional calculations using correlation-consistent basis sets as well as with MP2-R12 results.
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31.15.xp Perturbation theory
31.15.E- Density-functional theory
31.15.V- Electron correlation calculations for atoms, ions and molecules

Linear scaling calculation of maximally localized Wannier functions with atomic basis set

H. J. Xiang, Zhenyu Li, W. Z. Liang, Jinlong Yang, J. G. Hou, and Qingshi Zhu

J. Chem. Phys. 124, 234108 (2006); http://dx.doi.org/10.1063/1.2207622 (4 pages)

Online Publication Date: 20 June 2006

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We have developed a linear scaling algorithm for calculating maximally localized Wannier functions (MLWFs) using atomic orbital basis. An O(N) ground state calculation is carried out to get the density matrix (DM). Through a projection of the DM onto atomic orbitals and a subsequent O(N) orthogonalization, we obtain initial orthogonal localized orbitals. These orbitals can be maximally localized in linear scaling by simple Jacobi sweeps. Our O(N) method is validated by applying it to water molecule and wurtzite ZnO. The linear scaling behavior of the new method is demonstrated by computing the MLWFs of boron nitride nanotubes.
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71.15.-m Methods of electronic structure calculations

A convenient decontraction procedure of internally contracted state-specific multireference algorithms

Celestino Angeli, Carmen J. Calzado, Renzo Cimiraglia, and Jean-Paul Malrieu

J. Chem. Phys. 124, 234109 (2006); http://dx.doi.org/10.1063/1.2202738 (15 pages) | Cited 11 times

Online Publication Date: 20 June 2006

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Internally contracted state-specific multireference (MR) algorithms, either perturbative such as CASPT2 or NEVPT2, or nonperturbative such as contracted MR configuration interaction or MR coupled cluster, are computationally efficient but they may suffer from the internal contraction of the wave function in the reference space. The use of a low dimensional multistate model space only offers limited flexibility and is not always practicable. The present paper suggests a convenient state-specific procedure to decontract the reference part of the wave function from a series of state-specific calculations using slightly perturbed zero-order wave functions. The method provides an orthogonal valence bond reading of the ground state and an effective valence Hamiltonian, the excited roots of which are shown to be relevant. The orthogonal valence bond functions can be considered quasidiabatic states and the effective valence Hamiltonian gives therefore the quasidiabatic energies and the electronic coupling among the quasidiabatic states. The efficiency of the method is illustrated in two case problems where the dynamical correlation plays a crucial role, namely, the LiF neutral/ionic avoided crossing and the F2 ground state wave function.
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31.15.xp Perturbation theory
31.15.bw Coupled-cluster theory
31.15.ve Electron correlation calculations for atoms and ions: ground state
33.80.Be Level crossing and optical pumping

Graph transformation method for calculating waiting times in Markov chains

Semen A. Trygubenko and David J. Wales

J. Chem. Phys. 124, 234110 (2006); http://dx.doi.org/10.1063/1.2198806 (16 pages) | Cited 8 times

Online Publication Date: 21 June 2006

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We describe an exact approach for calculating transition probabilities and waiting times in finite-state discrete-time Markov processes. All the states and the rules for transitions between them must be known in advance. We can then calculate averages over a given ensemble of paths for both additive and multiplicative properties in a nonstochastic and noniterative fashion. In particular, we can calculate the mean first-passage time between arbitrary groups of stationary points for discrete path sampling databases, and hence extract phenomenological rate constants. We present a number of examples to demonstrate the efficiency and robustness of this approach.
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05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
02.50.Ga Markov processes
02.50.Cw Probability theory
02.10.Ox Combinatorics; graph theory

Nonuniversality of commonly used correlation-energy density functionals

Jacob Katriel, Sudip Roy, and Michael Springborg

J. Chem. Phys. 124, 234111 (2006); http://dx.doi.org/10.1063/1.2206183 (11 pages) | Cited 4 times

Online Publication Date: 21 June 2006

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The correlation energies of the helium isoelectronic sequence and of Hooke’s atom isoelectronic sequence have been evaluated using an assortment of local, gradient, and metagradient density functionals. The results are compared with the exact correlation energies, showing that while several of the more recent density functionals reproduce the exact correlation energies of the helium isoelectronic sequence rather closely, none is satisfactory for Hooke’s atom isoelectronic sequence. It is argued that the uniformly acceptable results for the helium sequence can be explained through simple scaling arguments that do not hold for Hooke’s atom sequence, so that the latter system provides a more sensitive testing ground for approximate density functionals. This state of affairs calls for further effort towards formulating correlation-energy density functionals that would be truly universal at least for spherically symmetric two-fermion systems.
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31.15.E- Density-functional theory
05.30.Fk Fermion systems and electron gas
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Anomalous oxygen isotope enrichment in CO2 produced from O+CO: Estimates based on experimental results and model predictions

Antra Pandey and S. K. Bhattacharya

J. Chem. Phys. 124, 234301 (2006); http://dx.doi.org/10.1063/1.2206584 (13 pages) | Cited 6 times

Online Publication Date: 15 June 2006

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The oxygen isotope fractionation associated with O+CO→CO2 reaction was investigated experimentally where the oxygen atom was derived from ozone or oxygen photolysis. The isotopic composition of the product CO2 was analyzed by mass spectrometry. A kinetic model was used to calculate the expected CO2 composition based on available reaction rates and their modifications for isotopic variants of the participating molecules. A comparison of the two (experimental data and model predictions) shows that the product CO2 is endowed with an anomalous enrichment of heavy oxygen isotopes. The enrichment is similar to that observed earlier in case of O3 produced by O+O2 reaction and varies from 70‰ to 136‰ for math and 41‰ to 83‰ for math. Cross plot of δ math and δ math of CO2 shows a linear relation with slope of ∼ 0.90 for different experimental configurations. The enrichment observed in CO2 does not depend on the isotopic composition of the O atom or the sources from which it is produced. A plot of Δ(δ math) versus Δ(δ math) (two enrichments) shows linear correlation with the best fit line having a slope of ∼ 0.8. As in case of ozone, this anomalous enrichment can be explained by invoking the concept of differential randomization/stabilization time scale for two types of intermediate transition complex which forms symmetric (mathmathmath) molecule in one case and asymmetric (mathmathmath and mathmathmath) molecules in the other. The δ math value of CO2 is also found to be different from that of the initial CO due to the mass dependent fractionation processes that occur in the O+CO→CO2 reaction. Negative values of Δ(δ math) ( ∼ 12.1‰) occur due to the preference of math in CO2* formation and stabilization. By contrast, at lower pressures ( ∼ 100 torr) surface induced deactivation makes Δ(δ math) zero or slightly positive.
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82.30.Nr Association, addition, insertion, cluster formation
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)

Specific and nonspecific interactions in a molecule with flexible side chain: 2-phenylethanol and its 1:1 complex with argon studied by high-resolution UV spectroscopy

S. Chervenkov, R. Karaminkov, J. E. Braun, H. J. Neusser, Sujit S. Panja, and Tapas Chakraborty

J. Chem. Phys. 124, 234302 (2006); http://dx.doi.org/10.1063/1.2202831 (11 pages) | Cited 8 times

Online Publication Date: 15 June 2006

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Using high-resolution resonance-enhanced two-photon ionization spectroscopy in combination with genetic-algorithm-based computer-aided rotational fit analysis and ab initio quantum chemistry calculations we determined the conformational structure and transition moment orientation in 2-phenylethanol and its 1:1 clusters with argon. The results clearly demonstrate that the gauche structure of 2-phenylethanol, which is stabilized by the intramolecular π-hydrogen bond between the folded side chain and the benzene ring, is the most abundant in the cold molecular beam. In this conformer the transition moment is rotated by 18° from the short axis of the aromatic ring. Two distinct 1:1 complexes of 2-phenylethanol with argon in a cis- and trans-configuration with respect to the side chain have been found. Employing the Kraitchman [Am. J. Phys. 21, 17 (1953) ] analysis we have found that the structure of the 2-phenylethanol moiety and the orientation of the transition moment do not change after the complexation with argon within the experimental accuracy. From the measured band intensities we conclude that in addition to the dispersion interaction of the argon atom with the aromatic ring a hydrogen-bond-type interaction with the terminal –OH group of the side chain stabilizes the cis-structure of the 1:1 complex of 2-phenylethanol with argon.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
31.15.A- Ab initio calculations
33.15.Bh General molecular conformation and symmetry; stereochemistry

On a singularity-free pair-polarizability anisotropy model for atomic gases

M. Chrysos, S. Dixneuf, and F. Rachet

J. Chem. Phys. 124, 234303 (2006); http://dx.doi.org/10.1063/1.2210478 (4 pages)

Online Publication Date: 16 June 2006

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We present a collision-induced light scattering spectrum of Ne2 and analyze it, together with an induced spectrum of Ar2, in terms of a model for the pair-polarizability anisotropy β of an atomic gas. This model, which is in essence a slight modification of one introduced by Ceccherini et al. [J. Chem. Phys. 111, 6316 (1999) ], is shown to describe the measured spectra much farther into the wings than does the original model. The agreement of our measurements with quantum-mechanical spectra from the most reliable, now available, ab initio computation methods for β produces evidence in support of the data and counters criticism of our experiment.
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33.20.Fb Raman and Rayleigh spectra (including optical scattering)
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
31.15.A- Ab initio calculations

Electronic structure calculations on the C4 cluster

H. Massó, M. L. Senent, P. Rosmus, and M. Hochlaf

J. Chem. Phys. 124, 234304 (2006); http://dx.doi.org/10.1063/1.2187972 (8 pages) | Cited 21 times

Online Publication Date: 16 June 2006

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The ground and the electronically excited states of the C4 radical are studied using interaction configuration methods and large basis sets. Apart from the known isomers [l-C4(X math) and r-C4(X math)], it is found that the ground singlet surface has two other stationary points: s-C4(X math) and d-C4(X math). The d-C4 form is the third isomer of this cluster. The isomerization pathways from one form to the other show that deep potential wells are separating each minimum. Multireference configuration interaction studies of the electronic excited states reveal a high density of electronic states of these species in the 0–2 eV energy ranges. The high rovibrational levels of l-C4(math) undergo predissociation processes via spin-orbit interactions with the neighboring math state.
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36.40.Cg Electronic and magnetic properties of clusters
36.40.Jn Reactivity of clusters
36.40.Qv Stability and fragmentation of clusters
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
82.30.Qt Isomerization and rearrangement
33.20.Vq Vibration-rotation analysis

Density functional theory study of self-association of N-methylformamide and its effect on intramolecular and intermolecular geometrical parameters and the cis/trans population

A. García Martínez, E. Teso Vilar, A. García Fraile, and P. Martínez-Ruiz

J. Chem. Phys. 124, 234305 (2006); http://dx.doi.org/10.1063/1.2204910 (6 pages) | Cited 1 time

Online Publication Date: 16 June 2006

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The single-point total energy (E) of several acyclic and cyclic oligomers of N-methylformamide (NMF) was computed by the first time without any geometrical restriction, using the B3LYP/6-31G* method of the density functional theory in order to determine the effect of self-association on intramolecular geometrical parameters of cis- and trans-NMF, the intermolecular distances of the hydrogen-bonding chains formed by NMF as well as intermolecular association energies including couterpoise corrections. It is concluded that liquid NMF exists mainly as polymers formed by self-association of trans-NMF units, whereas the cis-NMF isomer occurs as isolated units inserted along the chains. These computational results are in accordance with the experimentally determined predominance (ca. 90%) of trans-NMF population by means of math- NMR and other spectroscopic techniques, but in severe contradiction with a recent interpretation of x-ray diffraction data on liquid NMF, postulating a cyclic trimer of cis-NMF (c-C3) as the predominating species. The counterpoise-corrected values of the association energy, ΔE(CP), calculated for cyclic oligomers, increase with the polymerization degree (n) revealing a high grade of cooperative effect for amidic hydrogen-bonded chains. Noteworthy, the difference between the ΔE(CP) values of the cyclic cis- and trans-homooligomers of NMF is positive for n = 2 and 3 but negative for n ≥ 4.
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82.30.Nr Association, addition, insertion, cluster formation
31.15.E- Density-functional theory
82.35.-x Polymers: properties; reactions; polymerization

Inner-valence states of N2+ and the dissociation dynamics studied by threshold photoelectron spectroscopy and configuration interaction calculation

Tomohiro Aoto, Kenji Ito, Yasumasa Hikosaka, Akihiro Shibasaki, Ryo Hirayama, Norifumi Yamamono, and Eisaku Miyoshi

J. Chem. Phys. 124, 234306 (2006); http://dx.doi.org/10.1063/1.2206586 (6 pages) | Cited 13 times

Online Publication Date: 16 June 2006

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The N2+ states lying in the ionization region of 26–45 eV and the dissociation dynamics are investigated by high-resolution threshold photoelectron spectroscopy and threshold photoelectron-photoion coincidence spectroscopy. The threshold photoelectron spectrum exhibits several broad bands as well as sharp peaks. The band features are assigned to the N2+ states associated with the removal of an inner-valence electron, by a comparison with a configuration interaction calculation. In contrast, most of the sharp peaks on the threshold photoelectron spectrum are allocated to ionic Rydberg states converging to N22+. Dissociation products formed from the inner-valence N2+ states are determined by threshold photoelectron-photoion coincidence spectroscopy. The dissociation dynamics of the inner-valence ionic states is discussed with reference to the potential energy curves calculated.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.60.+q Photoelectron spectra
31.15.vn Electron correlation calculations for diatomic molecules
82.50.Hp Processes caused by visible and UV light
31.50.-x Potential energy surfaces

Theoretical investigation of RbCs via two-component spin-orbit pseudopotentials: Spectroscopic constants and permanent dipole moment functions

Ivan S. Lim, Won Chai Lee, Yoon Sup Lee, and Gwang-Hi Jeung

J. Chem. Phys. 124, 234307 (2006); http://dx.doi.org/10.1063/1.2204607 (12 pages) | Cited 5 times

Online Publication Date: 16 June 2006

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Potential energy curves for the 28 lowest ΛΣ states and 49 Ω states of RbCs are obtained from large-scale multireference configuration interaction calculations using both spin-averaged and two-component spin-orbit energy-consistent effective core potentials. Spectroscopic properties of all states are compared across available data in literature to date. Variations of the permanent dipole moments on the internuclear separation (R) for the math, math, math, and math states are evaluated over a wide range of R. The most important effects of the spin-orbit interaction on the dipole moment distribution are discussed.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
31.50.-x Potential energy surfaces
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Temperature- and species-dependent quenching of NO A math(v′ = 0) probed by two-photon laser-induced fluorescence using a picosecond laser

Thomas B. Settersten, Brian D. Patterson, and Jeffrey A. Gray

J. Chem. Phys. 124, 234308 (2006); http://dx.doi.org/10.1063/1.2206783 (14 pages) | Cited 14 times

Online Publication Date: 19 June 2006

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We report improved measurements of the temperature-dependent cross sections for the quenching of fluorescence from the A math(v′ = 0) state of NO. Cross sections were measured for gas temperatures ranging from 294 to 1300 K for quenching by NO(X math), H2O, CO2, O2, CO, N2, and C2H2. The A math(v′ = 0) state was populated via two-photon excitation with a picosecond laser at 454 nm, and the decay rate of the fluorescence originating from A math(v′ = 0) was measured directly. Thermally averaged quenching cross sections were determined from the dependence of the fluorescence decay rate on the quencher gas pressure. Our measurements are compared to previous measurements and models of the quenching cross sections, and new empirical fits to the data are presented. Our new cross-section data enable predictions in excellent agreement with prior measurements of the fluorescence lifetime in an atmospheric-pressure methane-air diffusion flame. The agreement resolves discrepancies between the lifetime measurements and predictions based on the previous quenching models, primarily through improved models for the quenching by H2O, CO2, and O2 at temperatures less than 1300 K.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.50.Dq Fluorescence and phosphorescence spectra

The rotational and fine-structure spectrum of FeH, studied by far-infrared laser magnetic resonance

John M. Brown, Helga Körsgen, Stuart P. Beaton, and Kenneth M. Evenson

J. Chem. Phys. 124, 234309 (2006); http://dx.doi.org/10.1063/1.2198843 (10 pages) | Cited 9 times

Online Publication Date: 20 June 2006

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Transitions between the spin-rotational levels of the FeH radical in the υ = 0 level of the X math ground state have been detected by the technique of laser magnetic resonance at far-infrared wavelengths. Both pure rotational and fine-structure transitions have been observed; lambda-type doubling is resolved on all the observed transitions. The energy levels of FeH are strongly affected by the breakdown of the Born-Oppenheimer approximation and cannot be modeled accurately by an effective Hamiltonian. The data are therefore fitted to an empirical formula to yield term values and g factors for the various spin-rotational levels involved. Many of the resonances show a doubling that arises from the proton hyperfine structure. These splittings are analyzed in a similar manner.
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33.35.+r Electron resonance and relaxation
33.20.Sn Rotational analysis
33.15.Pw Fine and hyperfine structure
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions

Vibrational shifts of OCS in mixed clusters of parahydrogen and helium

F. Paesani and K. B. Whaley

J. Chem. Phys. 124, 234310 (2006); http://dx.doi.org/10.1063/1.2202318 (13 pages) | Cited 11 times

Online Publication Date: 20 June 2006

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We present a detailed theoretical study of the solvation structure and solvent induced vibrational shifts for an OCS molecule embedded in pure parahydrogen clusters and in mixed parahydrogen/helium clusters. The use of two recent OCS-(parahydrogen) and OCS-helium ab initio potential energy surfaces having explicit dependence on the asymmetric stretch of the OCS molecule allows calculation of the frequency shift of the OCS ν3 vibration as a function of the cluster size and composition. We present results for clusters containing up to a full first solvation shell of parahydrogen (N = 17 molecules), and up to M = 128−N helium atoms. Due to the greater interaction strength of parahydrogen than helium with OCS, in the mixed clusters the parahydrogen molecules always displace He atoms in the first solvation shell around OCS and form multiple axial rings as in the pure parahydrogen clusters. In the pure clusters, the chemical potential of parahydrogen shows several magic numbers (N = 8,11,14) that reflect an enhanced stability of axial rings containing one less molecule than required for complete filling at N = 17. Only the N = 14 magic number survives in the mixed clusters, as a result of different filling orders of the rings and greater delocalization of both components. The OCS vibration shows a redshift in both pure and mixed clusters, with N-dependent values that are in good agreement with the available experimental data. The dependence of the frequency shift on the cluster size and its composition is analyzed in terms of the parahydrogen and helium density distributions around the OCS molecule as a function of N and M. The frequency shift is found to be strongly dependent on the detailed distribution of the parahydrogen molecules in the pure parahydrogen clusters, and to be larger but show a smoother dependence on N in the presence of additional helium, consistent with the more delocalized nature of the mixed clusters.
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31.15.A- Ab initio calculations
31.50.-x Potential energy surfaces
33.15.Mt Rotation, vibration, and vibration-rotation constants
36.40.-c Atomic and molecular clusters
33.20.Tp Vibrational analysis

A global 12-dimensional ab initio potential energy surface and dynamical studies for the SiH4+H→SiH3+H2 reaction

Manhui Wang, Xiaomin Sun, Wensheng Bian, and Zhengting Cai

J. Chem. Phys. 124, 234311 (2006); http://dx.doi.org/10.1063/1.2203610 (9 pages) | Cited 5 times

Online Publication Date: 21 June 2006

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A global 12-dimensional ab initio interpolated potential energy surface (PES) for the SiH4+H→SiH3+H2 reaction is presented. The ab initio calculations are based on the unrestricted quadratic configuration interaction treatment with all single and double excitations together with the cc-pVTZ basis set, and the modified Shepard interpolation method of Collins and co-workers [ K. C. Thompson et al., J. Chem. Phys. 108, 8302 (1998) ; M. A. Collins, Theor. Chem. Acc. 108, 313 (2002) ; R. P. A. Bettens and M. A. Collins, J. Chem. Phys. 111, 816 (1999) ] is applied. Using this PES, classical trajectory and variational transition state theory calculations have been carried out, and the computed rate constants are in good agreement with the available experimental data.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Fd Collision theories; trajectory models
82.20.Pm Rate constants, reaction cross sections, and activation energies
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