Franklin B. Brown

A double many‐body expansion of the two lowest‐energy potential surfaces and nonadiabatic coupling for H3

We present a consistent analytic representation of the two lowest potential energy surfaces for H3 and their nonadiabatic coupling. The surfaces are fits to ab initio calculations published previously by Liu and Siegbahn and also to new ab initio calculations reported here. The analytic representations are especially designed to be valid in the vicinity of the conical intersection of the two lowest surfaces, at geometries important for the H+H2 reaction, and in the van der Waals regions.

A new semi-empirical method of correcting large-scale configuration interaction calculations for incomplete dynamic correlation of electrons

Abstract We show that large-basis-set configuration interaction calculations including all single and double excitations from a complete-active-space optimized by MCSCF methods recover an approximately constant fraction of the external correlation energy. Therefore, by scaling this contribution to the energy, more accurate potential energy curves, barrier heights to chemical reactions, and potential energy surfaces can be calculated.

An improved potential energy surface for F+H2→HF+H and H+H′F→HF+H′

We present an improved analytic potential energy surface for the F+H2→FH+H and H+FH′→HF+H′ reactions. The final surface is obtained in two stages. First we create a surface, called No. 4, which is based in the F–H–H barrier region on a previous partly empirical and partly theoretical fit and is based on the F–H⋅⋅⋅H exit channel and H–F–H barrier regions on new large‐basis‐set configuration interaction calculations. The final surface, called No. 5 incorporates more empirical information for collinear geometries in both the F–H–H and FH⋅⋅⋅H regions but remains a good representation of the ab initio calculations for bending potentials and in the strong‐interaction regions. Variational‐transition‐state theory rate constants and WKB adiabatic barrier heights indicate that the final surface is more accurate than previous surfaces for thermal rate constants and overall reaction thresholds for F+H2→HF+H, F+D2→DF+D, and F+HD→HF+D and for product‐state thresholds for HF (n′=3) and DF(n′=4), where n′ is the final vi...

Global potential‐energy surfaces for H2Cl

We present two new analytic potential‐energy surfaces suitable for studying the competition between the abstraction reaction H+DCl→HD+Cl and the exchange reaction H+DCl→HCl+D. In the abstraction channel the surfaces are only slightly different from the Stern–Persky–Klein GSW surface, but the exchange barrier on both surfaces is raised by inclusion of a three‐center term fitted to ab initio extended‐basis‐set multireference configuration interaction calculations with scaled external correlation. The two surfaces differ significantly only for the steepness of H–Cl–H bend potential. The exchange and abstraction saddle points are characterized by harmonic analysis for H2Cl, HDCl, and D2Cl, and we also compute vibrationally adiabatic barrier heights including anharmonicity. We also report thermal rate constants and activation energies for both reactions mentioned above.

The potential energy surface for the F+H2 reaction as a function of bond angle in the saddle point vicinity

We report large‐basis‐set CASSCF/MR‐CISD/SEC (complete active space self‐consistent‐field orbitals used for multireference configuration interaction with all single and double excitations and scaled external correlation) and MP4 (Mo/ller–Plesset fourth order perturbation theory) calculations of the FH2 potential energy surface for collinear and bent geometries in the vicinity of the F‐‐‐H‐‐H saddle point. These calculations indicate that higher order correlation effects become much more important as the generalized transition states are bent, and that the unrestricted saddle point for this reaction is noncollinear. This means that the sterically allowed cone of reactive configurations is much broader than either previously available ab initio calculations or the present lower‐order ones would predict.

Dissociation potential for breaking a CH bond in methane

The potential energy along the dissociation coordinate for CH4 → CH3 + H is calculated by the multi-reference configuration interaction method, including all single and double excitations, with a triple-split-plus-diffuse-and-extended-polarization basis set. The three-parameter Lippincott and Varshni potential curves provide better approximations to the dissociation potential than does the Morse model.

An improved calculation of the transition state for the F+H2 reaction

Abstract Using a large, balanced one-electron basis set, fully optimized reaction space (FORS) calculations to optimize the orbitals and to estimate the internal correlation energy, multi-reference configuration interaction calculations including all single and double excitations out of the FORS reference space to estimate a fraction of the external correlation energy, and the method of scaled external correlation (SEC), we calculate the interaction energy of F with H 2 in the vicinity of the saddle point for the reaction F + H 2 → HF + H. Our calculated barrier height, 1.6 kcal/mol, is considerably lower than values obtained in recent ab initio calculations, and the saddle point geometry is about 0.3 a 0 looser. This indicates that the part of the external correlation energy omitted from MR CISD calculations because of the incompleteness of the one-electron basis set and the truncation of the CI expansion, as estimated by the SEC method, has a significant effect on both the saddle point energy and its geometry.

A comparative study of potential energy surfaces for CH3+H2↔CH4+H

We analyze potential energy surfaces that have been proposed by one of the authors, Bunker, and Chapman and by Raff for the reaction CH3+H2↔CH4+H. The surfaces are modified to remove discontinuities and zero frequencies, where present, and the modified surfaces are compared to each other in terms of reaction‐path properties and to ab initio calculations for stationary point properties. They are also used for rate constant calculations which are compared to experiment. The rate constants were calculated by improved canonical variational transition state theory with small‐curvature semiclassical adiabatic ground‐state transmission coefficients (ICVT/SCSAG) over a wide temperature range, 298–1340 K. Both surfaces yield rate constants in poor agreement with experimental values. The reaction‐path analysis leads to a list of potential energy surface features that are important for the rate constants but inaccurate in the existing surfaces and that should be improved in subsequent work.

Reaction-path analysis of the tunneling splitting in fluxional molecules: Application to the degenerate rearrangement of hydrogen fluoride dimer

The small‐curvature semiclassical adiabatic (SCSA) approximation, which is based on a reaction‐path Hamiltonian, is used to calculate the tunneling splitting due to the degenerate rearrangement of hydrogen fluoride dimer. The calculation employs a semiempirical potential energy surface which approximates the HF molecules as rigid rotators, and for which accurate tunneling splittings have been previously calculated. The semiclassical method is shown to be accurate within 33%. The internal motion of the dimer along the reaction path and the contributions of the generalized normal mode vibrations to reaction‐path curvature in the tunneling region are also discussed.

Semiclassical reaction‐path methods applied to calculate the tunneling splitting in ammonia

The small‐curvature semiclassical adiabatic (SCSA) approximation, which is based on a reaction‐path Hamiltonian, is used to calculate the rate of interconversion of the ammonia invertomers. In these calculations, we employ two realistic potential energy surfaces for which accurate tunneling splittings have been calculated previously. The semiclassical method is shown to be accurate within 16% for this prototype quantal isomerization rate.