We introduce TheRate (THEoretical RATEs), a complete application program with a graphical user interface (GUI) for calculating rate constants from first principles. It is based on canonical variational transition‐state theory (CVT) augmented by multidimensional semiclassical zero and small curvature tunneling approximations. Conventional transition‐state theory (TST) with one‐dimensional Wigner or Eckart tunneling corrections is also available. Potential energy information needed for the rate calculations are obtained from ab initio molecular orbital and/or density functional electronic structure theory. Vibrational‐state‐selected rate constants may be calculated using a diabetic model. TheRate also introduces several technical advancements, namely the focusing technique and energy interpolation procedure. The focusing technique minimizes the number of Hessian calculations required by distributing more Hessian grid points in regions that are critical to the CVT and tunneling calculations and fewer Hessian grid points elsewhere. The energy interpolation procedure allows the use of a computationally less demanding electronic structure theory such as DFT to calculate the Hessians and geometries, while the energetics can be improved by performing a small number of single‐point energy calculations along the MEP at a more accurate level of theory. The CH4+H↔CH3+H2 reaction is used as a model to demonstrate usage of the program, and the convergence of the rate constants with respect to the number of electronic structure calculations. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1039–1052, 1998

Erratum: "Thermal and vibrational-state selected rates of the reaction" [J. Chem. Phys. 103, 9642 (1995)]

We present direct ab initio dynamics studies of thermal and vibrational-state selected rates of the hydrogen abstraction CH4+Cl↔CH3+HCl reaction. Rate constants were calculated within the canonical variational transition state theory formalism augmented by multidimensional semiclassical tunneling corrections. A vibrational diabatic model was used for vibrational-state selected rate calculations, particularly for exciting the CH4 symmetric stretching and umbrella bending modes. The potential energy information was calculated by a combined density functional and molecular orbital approach. Becke’s half-and-half (BH&H) nonlocal exchange and Lee–Yang–Parr (LYP) nonlocal correlation functionals (BH&HLYP) were used with the 6-311G(d,p) basis set for determining structures and frequencies at the stationary points and along the minimum energy path (MEP). Energetics information was further improved by a series of single point spin-projected fourth-order Mo/ller–Plesset perturbation theory (PMP4(SDTQ)) calculations using the 6-311+G(2df,2pd) basis set. We found that the calculated thermal rate constants have reasonable agreement with experimental results for both the forward and reverse reactions. Our results also predict that exciting the CH4symmetric stretching mode will greatly enhance the hydrogen atom transfer rate. Surprisingly, exciting the CH4 umbrella bend mode is also predicted to have a noticeable enhancement factor at room temperature.

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We present a new direct ab initio dynamics methodology for calculating thermal rate constants from density functional theory (DFT). Dynamical theory is based on a full variational transition state theory plus multidimensional semiclassical tunneling approximations. We have applied this approach to the CH3+H2→CH4+H abstraction reaction using the BH&H-LYP method which is the combination of the hybrid Becke’s half-and-half (BH&H) method for nonlocal exchange and Lee–Yang–Parr (LYP) functional for nonlocal correlation. The 6-311G(d,p) basis set was used in these calculations. To obtain quantitative results, the classical potential energy along the minimum energy path (MEP) was corrected either by scaling to match a more accurate ab initio results for the barrier heights or by carrying out single point calculations at selected points along the MEP at a more accurate level of ab initio molecular orbital (MO) theory. By comparing with our previous QCISD results and experimental rate constants, we found that DFT particular the BH&H-LYP method can provide sufficient accurate potential energy surface information for rate calculations for this system. The present direct DFT dynamics method can be used for reactive dynamics studies of reactions involving large polyatomic molecules from first principles. More work however is still needed to test the accuracy of DFT methods for such calculations.

A direct ab initio dynamics study on the gas-phase reactions of atomic hydrogen with different fluoromethanes has been carried out. The thermal rate constants were calculated using canonical variational transition state (CVT) theory augmented by multidimensional semiclassical zero and small curvature tunneling approximations. The potential energy surfaces for the reactions were calculated using hybrid density functional theory, namely, Becke's half-and-half (BH) nonlocal exchange and the Lee-Yang-Parr (LYP) nonlocal correlation functionals using the cc-pVDZ basis set. The reaction energies and barrier heights were improved by single-point energy calculations along the minimum energy path (MEP) at the spin-projected fourth order Moller-Plesset perturbation theory (PMP4) using the cc-pVTZ basis set. The calculated forward and reverse thermal rate constants are in the good agreement with the experimental data. The electronic effects of fluorine substitution on the rate of this class of reactions are examined.

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