The frozen-core approximation for diatomic molecules: The lowest ¹ Σ ⁺ states of H₂

Adiabatic potential energies for the two lowest ¹ Sigma ⁺ states of the hydrogen molecule have been calculated in the frozen core approximation over a range of internuclear separations from 0.2 to 4 au. The results are comparable with the SCF calculation of Coulson (1938) for the X¹ Sigma _g⁺ ground state, and with the multi-configuration calculation of Browne (1969) for the B¹ Sigma _u⁺ excited state. Transition moments for the X¹ Sigma _g⁺-B¹ Sigma _u⁺ electronic transition have also been computed.