On the coupling strength in potential energy surfaces for vibrational calculations

A measure of the importance of two- and three-mode couplings in molecular potential energy surfaces is introduced based on perturbation theory arguments. The measure requires a low-order polynomial representation of the potential energy function. The method is appropriate for reducing the computational cost of potential energy surface generation as well as vibrational wave function calculations. Example applications on grid-based potentials for formaldehyde and ethylene oxide are reported.