Para–ortho isomerization of H2+. Non-Born–Oppenheimer direct variational calculations with explicitly correlated all-particle Gaussian functions

Abstract

Direct variational calculations are performed for all bound rovibrational states of the H2+ ion corresponding to the ground and first excited rotational levels (the N=0 and N=1 states). The Born–Oppenheimer (BO) approximation is not assumed in the calculations and all-particle explicitly correlated Gaussian basis functions are used for the wave-function expansion. The exponential parameters of the Gaussians are optimized with the aid of analytically calculated energy gradient determined with respect to these parameters. The non-BO energies are used to determine the ortho–para nuclear-spin isomerization energies and the non-BO wave functions are used to determine the expectation values of the interparticle distances.