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⁺ (^3,4He^1,2,3H⁺), 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.