Simultaneous determination of nuclear and electronic wave functions without Born-Oppenheimer approximation: Ab initio NO+MO/HF theory

We develop a simultaneous determination method of nuclear and electronic wave functions without the Born-Oppenheimer approximation. We examine two expanding methods, namely, molecular orbital (MO)-type and valence bond (VB)-type expansions for a nuclear orbital, which is a one-particle wave function of a nucleus. The VB-type expansion is shown to be more accurate than the MO-type one because of the local nature of the nuclei. We also investigate the basis function expansion of the nuclear orbital and propose a scheme to determine the orbital exponent for the nuclear basis function. Numerical calculations confirm the accuracy and feasibility of the present method.