The Möller–Plesset perturbation theory applied to the determination of non-rigid molecule vibrational levels

The accuracy of the Möller–Plesset perturbation theory for the determination of non-rigid molecule vibrational energy levels, is explored. As examples of effectiveness, a three-dimensional Hamiltonian of dimethyl-ether and a two-dimensional Hamiltonian of ethanol are solved self-consistently and the resulting levels are corrected by the perturbation theory. The method reduces significantly computational expenses and dimensionality and permits to calculate the lowest vibrational states and the tunneling splitting with enough accuracy. It fails in zones with high density of states. It can be applied for determining the levels of large molecules with many large amplitude vibrations and for introducing the interaction between low and high vibrational modes.