Rozeanne Steckler

POLYRATE 4: A new version of a computer program for the calculation of chemical reaction rates for polyatomics

POLYRATE is a computer program for the calculation of chemical reaction rates of polyatomic species (and also atoms and diatoms as special cases). Version 1.1 was submitted to the CPC Program Library in 1987, and version 4.0.1 was submitted in 1992. Since that time many new capabilities have been added, old ones have been improved, and the code has been made more portable and user-friendly, resulting in the present improved version 6.5. The methods used are variational or conventional transition state theory and multidimensional semiclassical adiabatic and large-curvature approximations for tunneling and nonclassical reflection. Rate constants may be calculated for canonical or microcanonical ensembles or for specific vibrational states of selected modes with translational, rotational, and other vibrational modes treated thermally. Bimolecular and unimolecular reactions and gas-phase, solid-state, and gas-solid interface reactions are all included. Potential energy surfaces may be global analytic functions or implicit functions defined by interpolation from input energies, gradients, and force constants (Hessian matrices) at selected points on a reaction path. The data needed for the dynamics calculations may also be calculated from a global potential energy surface with more accurate calculations at stationary points. The program calculates reaction paths by the Euler, Euler stabilization, or Page-McIver methods. Variational transition states are optimized from among a one-parameter sequence of generalized transition states orthogonal to the reaction path. Tunneling probabilities are calculated by numerical quadrature, using either the centrifugal-dominant-small-curvature approximation, the large-curvature-version-3 approximation, and/or optimized multidimensional tunneling approximations. In the large-curvature case the tunneling probabilities may be summed over final vibrational states for exoergic reactions or initial vibrational states for endoergic reactions.

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...

Variational transition state theory calculations of the reaction rates of F with H2, D2, and HD and the intermolecular and intramolecular kinetic isotope effects

We use variational transition state theory to calculate rate constants and kinetic isotope effects for the reactions F+H2→HF+H (with rate constant k1), F+D2→DF+D(k2), and two other isotopic analogs as functions of temperature. The calculations are performed using a recently proposed partly empirical, partly ab initio potential energy surface, called surface No. 5, and also using a new surface, called surface No. 5A, introduced here to test the effect of a higher classical saddle point on the reaction rates, kinetic isotope effects, and reaction thresholds. The various theoretical results are compared to the available experiments to test the validity of these potential energy surfaces. For those rate constants and kinetic isotope effects for which there is more than one experimental value at a given temperature, the theoretical results for reactions on surface No. 5 agree with experiment about as well as the individual experiments agree with each other. At T>373 K where there is only one experimental measu...

A new potential energy surface for the CH3+H2↔CH4+H reaction: Calibration and calculations of rate constants and kinetic isotope effects by variational transition state theory and semiclassical tunneling calculations

We present a sequence of three successively improved new semiempirical potential energy surfaces for the reaction CH3+H2→CH4+H. The semiempirical calibration is based on ab initio electronic structure calculations and experimental thermochemical data, vibrational frequencies, reaction rate constants, Arrhenius parameters, and kinetic isotope effects (KIE’s). To compare to the experimental kinetic data we apply variational transition state theory and semiclassical estimates of tunneling probabilities. We also provide detailed factorization analyses of the KIE’s to illustrate the way in which various surface features contribute to the overall KIE’s, and we discuss the substantial difficulties in attributing specific kinetic results to isolated potential energy surface features. Each of the three new surfaces, called J1, J2, and J3, has a thinner barrier than the one before. In addition, we provide one example, called surface J2A, showing the effect of making the barrier even thinner than on the best surface...

Use of scaled external correlation, a double many-body expansion, and variational transition state theory to calibrate a potential energy surface for FH2

A new potential energy surface is presented for the reaction F+H2→HF+H. The regions of the surface corresponding to collinear and bent geometries in the F–H–H and H–F–H barrier regions are based on scaled external correlation (SEC) electronic structure calculations, and the F–H⋅⋅⋅H exit channel region is based on the previously developed surface No. 5. The functional form of the new surface includes dispersion forces by a double many‐body expansion (DMBE), and the surface was adjusted so that the van der Waals well in the F⋅⋅⋅H–H region agrees with available experimental predictions. We have calculated stationary point properties for the new surface as well as product–valley barrier maxima of vibrationally adiabatic potential curves for F+H2→HF(v’=3)+H,F+HD→HF(v’=3)+D, and F+D2→DF(v’=4)+D. The new surface should prove useful for studying the effect on dynamics of a low, early barrier with a wide, flat bend potential, as indicated by the best available electronic structure calculations.

Transition state structure, barrier height, and vibrational frequencies for the reaction Cl+CH4→CH3+HCl

We have carried out ab initio calculations using second‐ and fourth‐order Mo/ller–Plesset perturbation theory, scaled electron correlation, and several basis sets for the reaction Cl+CH4→CH3+HCl. We found that including electron correlation is essential for obtaining accurate barrier heights and vibrational frequencies. Furthermore, scaling the correlation energy further improves the barrier height predictions provided that the basis set being used is correlation balanced for both bonds involved in the reaction. Geometries and transition state frequencies calculated at the MP2 and MP‐SAC2 levels with the most extensive and best balanced basis set are in good agreement with one another for all bound modes, but the unbound‐mode frequency changes by 214i cm−1.

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.

MORATE: a program for direct dynamics calculations of chemical reaction rates by semiempirical molecular orbital theory

Abstract We present a computer program, MORATE (Molecular Orbital RATE calculations), for direct dynamics calculations of unimolecular and bimolecular rate constants of gas-phase chemical reactions involving atoms, diatoms, or polyatomic species. The potential energies, gradients, and higher derivatives of the potential are calculated whenever needed by semiempirical molecular orbital theory without the intermediary of a global or semiglobal fit. The dynamical methods used are conventional or variational transition state theory and multidimensional semiclassical approximations for tunneling and nonclassical reflection. The computer program is conveniently interfaced package consisting of the POLYRATE program, version 4.5.1, for dynamical rate calculations, and the MOPAC program, version 5.03, for semiempirical electronic structure computations. All semiempirical methods available in MOPAC, in particular MINDO/3, MNDO, AM1, and PM3, can be called on to calculate the potential and gradient. Higher derivatives of the potential are obtained by numerical derivatives of the gradient. Variational transition states are found by a one-dimensional search of generalized-transition-state dividing surfaces perpendicular to the minimum-energy path, and tunneling probabilities are evaluated by numerical quadrature.

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.

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.