Ionization potentials of HCN and HNC by a Green's function method

The vertical valence ionization potentials of HCN and HNC have been calculated by a many-body Green's function method using extended basis sets including polarization functions. For HCN the agreement of the computed ionization potentials with experiment is very satisfactory. The ordering is 1π, 3σ, 2σ. The ionization potentials of HNC have not been measured yet. The calculated ordering is 3σ, 1π, 2σ. The electronic structure of the two molecules is seen to differ.