Nonrelativistic Nonadiabatic Corrections to the Rovibrational Energies of HeH+ Isotopologues

We present nonrelativistic finite nuclear mass corrections to the rovibrational energies of six isotopologues of HeH+ (3,4He1,2,3H+), calculated within the framework of nonadiabatic perturbation theory (NAPT) up to the second order in the electron-to-nucleus mass ratio. Corrections are expressed through adiabatic and nonadiabatic potentials, evaluated with high numerical precision using explicitly correlated Kołos–Wolniewicz basis functions, suitable for heteronuclear systems with unequal nuclear charges. The potentials were determined over a broad range of internuclear distances, with accuracy close to the MHz level in the nonrelativistic dissociation energies of low-lying rovibrational states. For the ground level, the results were independently verified by variational calculations with the nonadiabatic James–Coolidge wave functions. The reported results establish a robust foundation for the spectroscopic precision modeling of HeH+, including an accurate treatment of relativistic and QED effects.