THE DEFINITION OF REACTION COORDINATES FOR REACTION-PATH DYNAMICS

We present equations for generalized-normal-mode vibrational frequencies in reaction-path calculations based on various sets of coordinates for describing the internal motions of the system in the vicinity of a reaction path. We consider two special cases in detail as examples, in particular three-dimensional atom-diatom collisions with collinear steepest descent paths and reactions of the form CX3 + YZ --> CX3Y + Z with reaction paths having C3-upsilon symmetry. We then present numerical comparisons of the differences in harmonic reaction-path frequencies for various coordinate choices for three such systems, namely, H + H-2 --> H-2 + H, O + H-2 --> OH + H, and CH3 + H-2 --> CH4 + H. We test the importance of the differences in the harmonic frequencies for dynamics calculations by using them to compute thermal rate constants using variational transition state theory with semiclassical ground-state tunneling corrections. We present a new coordinate system for the reaction CH3 + H-2 that should allow for more accurate calculations than the Cartesian system used for previous reaction-path calculations on this and other polyatomic systems.