Acknowledgment. We are grateful to the National Science Foundation for financial support of this research, the Office of Academic Computing at UCLA foruse of the computer facilities, and Professor Matthew Platz of The Ohio State University for
helpful discussions. Registry No. H CeH, 4218-50-2; H3CCcl, 31304-51-5; H3CeOH, 30967-49-8; H3CdOMe,65092-80-0; H3CCCH;ICHz, 98 115-38-9; HeCHzCH=CHz, 90566-94-2.
Reaction Path Following in Mass-Weighted Internal Coordinates
Carlos Gonzalez and H. Bernhard Schlegel*lt
Department o Chemistry, WayneState University, Detroit, Michigan 48202 (Received: November 1 , 1989; f In Final Form: April 3, 1990)
O r previous algorithm for following reaction paths downhill (J. Chem. Phys. 1989,90, 2154),has been extended to use u mass-weighted internal coordinates. Points on the reaction path are found by constrained optimizations involving the internal
degrees of freedom of the molecule. The pointsare optimized so that the segment of the reaction path between any two adjacent points is described by an arc of a circle in mass-weighted internal coordinates, and so that the gradients (inmass-weighted internals) at the end points of the arc are tangent to the path. The algorithm has the correct tangent vector and curvature vectors in the limit of small step size but requires only thetransition vector and the energy gradients; the resulting path is continuous, differentiable, and piecewise quadratic. Reactions paths for CH4 + H CHI + H1, HCN CNH, F + CH,F FCH3 + F, C2HSF C2H4+ HF arecalculated and the results are compared to the paths obtained with mass-weighted and Cartesians and with internal coordinates without mass-weighting.
1. Introduction The concept ofthe reaction pathway has become important in the study of potential energy surfaces for chemical reactions. In general, the reaction path can be defined as the curve on the potential energy surface...