THERATE - PROGRAM FOR AB-INITIO DIRECT DYNAMICS CALCULATIONS OF THERMAL AND VIBRATIONAL-STATE-SELECTED RATE CONSTANTS

We introduce TheRate (THEoretical RATEs), a complete application progr am with a graphical user interface (GUI) for calculating rate constant s from first principles. It is based on canonical variational transiti on-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 co rrections is also available. Potential energy information needed for t he rate calculations are obtained from db initio molecular orbital and /or density functional electronic structure theory. Vibrational-state- selected rate constants may be calculated using a diabetic model. TheR ate also introduces several technical advancements, namely the focusin g 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 en ergy interpolation procedure allows the use of a computationally less demanding electronic structure theory such as DFT to calculate the Hes sians and geometries, while the energetics can be improved by performi ng a small number of single-point energy calculations along the MEP at a more accurate level of theory. The CH4 + H <->CH3 + H-2 reaction is used as a model to demonstrate usage of the program, and the converge nce of the rate constants with respect to the number of electronic str ucture calculations. (C) 1998 John Wiley & Sons, Inc.