Bond-distance and bond-angle constraints in reaction-path dynamics calculations

Projection operator techniques for enforcing bond-distance and bond-angle constraints in reaction-path dynamics calculations are presented. These techniques provide a systematic method for carrying out reduced-dimensionality calculations in generalized normal mode coordinates with constrained values for selected internal coordinates. The methods are illustrated with generalized transition-state theory and multidimensional semiclassical tunneling calculations for the reactions OH + H₂ → H₂O + H and CH₃ + H₂ → CH₄ + H in which one or more bond lengths and/or bond angles is held fixed.