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

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.