REACTION-PATH FOLLOWING IN MASS-WEIGHTED INTERNAL COORDINATES

Our previous algorithm for following reaction paths downhill (J. Chem. Phys. 1989, 90, 2154), has been extended to use mass-weighted internal coordinates. Points on the reaction path are found by constrained optimizations involving the internal degrees of freedom of the molecule. The points are 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 (in mass-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 the transition vector and the energy gradients; the resulting path is continuous, differentiable, and piecewise quadratic. Reactions paths for CH4 + H → CH3 + H2, HCN → CNH, F- + CH3F → FCH3 + F-, and C2H5F → C2H4 + HF are calculated and the results are compared to the paths obtained with mass-weighted Cartesians and with internal coordinates without mass-weighting. © 1990 American Chemical Society.