Gerade-ungerade symmetry breaking in HD: States close to N=2 dissociation limit

We review the consequences of the gerade-ungerade symmetry breaking in HD molecule. A particular attention is devoted to electronic states close to n=2 dissociation limit. The effects of the breakdown of the Born-Oppenheimer approximation are discussed. For the description of the molecular dynamics the formulation of the coupled-sates method is given. The method is applied to calculations of loosely bound states in the outer well of the II'1Πg potential. The results are in excellent agreement with experimental data and calculations based on a semi-empirical potential.