Moller-Plesset perturbation theory for vibrational wave functions

The vibrational self-consistent field wave function is considered as the zeroth order state in a vibrational Moller-Plesset (VMP) perturbation theory. A method for calculating the contributions to arbitrary order is described and implemented. The theoretical background for understanding and analyzing the behavior of convergent and divergent VMP expansions is discussed briefly. Examples of convergent and divergent vibrational Moller-Plesset perturbation series are given and analyzed for two-mode model systems and for a formaldehyde quartic force field. It is found that direct use of high order VMP is problematic for calculation of anharmonic vibrational energies. (C) 2003 American Institute of Physics.