Gerade-ungerade symmetry breaking in HD: bound states supported by the I' (1)Pi(g) outer potential well

Gerade-ungerade symmetry breaking in HD for the bound states supported by the shallow outer I' (1)Pi(g) potential is studied theoretically. By clarifying the asymptotic behavior of the relevant nonadiabatic couplings among the stats correlating to the n=2 dissociation limit, simple two-state (for f-parity) and three-state (for e-parity) approximations are formulated. They reproduce binding energies in very good agreement with recent spectroscopic measurements. Comparisons with the calculations based on a single model potential are presented and the dependence of the results on the used ab initio Born-Oppenheimer (clamped nuclei) potentials is discussed.