Mo/ller–Plesset perturbation theory applied to vibrational problems

Abstract

Mo/ller–Plesset perturbation theory is employed to improve the accuracy of static mean field computations in molecular vibration problems. This method is a simple and efficient way to get nearly exact frequencies for few-mode model potentials. For more realistic potentials representing the dynamics of water and formaldehyde, the Mo/ller–Plesset treatment works equally as well. However, we find in general that MP2 level corrections give very accurate energies and additional corrections by higher level terms in the MP series are not substantial. Moreover, we find that for reference states on high energy manifolds degeneracies can result when higher level terms are included in the series. We discuss several ways to remove these degeneracies.