Abstract
It is easier to compute a vibrational spectrum when the potential energy surface (PES) is a sum-ofproducts
(SOP). Many popular computational methods work only if the PES is a SOP. However, the most
accurate PESs are not SOPs. We propose using collocation and solutions of the Schrödinger equation with
a SOP PES to compute solutions on a corresponding general PES. This makes it possible to account for
coupling and anharmonicity omitted from the SOP PES. We find that correcting energy levels computed on
a SOP PES with collocation reduces differences with exact energy levels by about two orders of magnitude.
