Configuration interaction calculations have been performed on the 3Πg symmetry of O2 which establish a general computational procedure for diabatic molecular Rydberg states. Three different calculations which include varying amounts of electronic correlation for the Rydberg and valence states are reported. The interaction potentials for the lowest diabatic Rydberg state from these three calculations are in excellent agreement showing that semi-internal correlation energy of Rydberg states has little effect on the potential curves. The Rydberg state is found to be exceedingly similar to the O2+2Πg state to which the 3Πg Rydberg series converges. Energies for the lowest four Rydberg vibrational levels are presented. © 1980 American Institute of Physics.