VARIATIONAL TRANSITION-STATE THEORY AND SEMICLASSICAL TUNNELING CALCULATIONS WITH INTERPOLATED CORRECTIONS - A NEW APPROACH TO INTERFACING ELECTRONIC-STRUCTURE THEORY AND DYNAMICS FOR ORGANIC-REACTIONS

In variational transition-state theory (VTST) and semiclassical tunnel ling calculations, especially those with semiempirical potential-energ y surfaces, it is sometimes desirable to match the classical energies and vibration frequencies of some points (e.g. the reactant, saddle po int, product, van der Waals complex, ion-molecule complex) along the m inimum-energy path (MEP) and in the reaction swath with high-level res ults, as this can improve the accuracy. This can be accomplished by ad ding a correction function to the calculated energies or frequencies. In this paper, we introduce a three-point or zero-order interpolated c orrection method which is based on the correction at three points, in particular the saddle point and two stationary points, one on each sid e of the MEP. We use the corrections at these points to build a correc tion function for the classical energy and for each vibrational mode f requency along the MEP. The function is calibrated such that the corre cted result matches the accurate values at these stationary points. Th e functional forms to be used depend on the shape of the MEP under con sideration and the relative correction values at those points. Similar treatments are applied to the determinant of the moment of inertia te nsor along the reaction path and to the potential-energy function in n on-adiabatic regions of corner-cutting tunnelling paths. Once paramete rs in the functional forms are determined, we then use the corrected e nergy, frequency and moments of inertia information together with othe r MEP and reaction swath data, as obtained directly from the potential -energy surface, to perform new VTST calculations. Details of the impl ementation are presented, and applications to reaction rate calculatio ns of the OH + CH4 --> H2O + CH3 and CF3 + CD3H --> CF3H + CD3 reactio ns are included.