MO theory of isotope symmetry breaking in HDO and HD

We report molecular orbital Hartree–Fock and post-Hartree–Fock adiabatic calculations of HDO and HD, so that the isotope effect is accounted for. The isotope symmetry breaking of HDO is predicted for the first time. The isotope shift of the states of HD converging to the n=2 separated atom level is obtained exactly. Mulliken population analysis illustrates the symmetry breaking phenomena, from D∞h to C∞v in HD and from C2v to C1 in HDO.