Local normal modes and vibrational adiabatic potentials

Local normal-mode analysis for a collinear potential energy surface generates a system of curvilinear coordinates, which are orthogonal in the mass-skewed system. The motion is locally separable in these coordinates. We compare the utility of one of the normal modes as a transversal-vibration coordinate, with the conventional choice of the direction perpendicular to the reaction coordinate in the mass-skewed system. The comparison is done for two commonly used reaction coordinates: BEBO and the steepest-descent path. Results differ for different choices of directions and reaction coordinates. Future work should concentrate on a choice of a reaction coordinate which is itself one of the normal modes.