Optimum choice of the reaction coordinate for adiabatic calculations of the tunneling probabilities

Abstract

The dependence of the harmonic vibrational frequencies on the choice of the reaction coordinate s off the reaction path raises serious concerns concerning mutual consistency of the adiabatic theory of chemical reactions when the activated complex is located away from the saddle point. The paper shows that use of the least-squares reaction coordinate allows one to eliminate linear terms in vibrational expansions of the coefficients of momentum coupling between s motion and any of vibrational degrees of freedom. It is argued that this feature of the least-squares reaction coordinate makes it preferable for adiabatic calculations of the tunneling probabilities.