Electronic Energy Levels of Molecular Oxygen

A configuration interaction study of the lower-lying electronic energy levels of molecular oxygen is made. With the 1s and 2s shells kept filled, the remaining eight electrons are distributed among the twelve spin-orbitals arising from the 2p levels on each atom. The atomic functions are represented as Gaussians, which are poor atomic functions but facile integrands in many-center integrals. The molecular orbitals are formed as symmetrical linear combinations of the atomic orbitals, simply for convenience. Inclusion of all configurations implies complete equivalence of all possible constructions of linear combinations. Out of the 495 configurations (many-electron determinants) a group of nine states of 3Σg− character is set up and twelve of 1Σg+ character. The interaction matrix of each block is computed and diagonalized (by electronic computer). The results for the ground state, which is a triplet, are very good. The binding energy is off by 2 percent, the internuclear distance is off by 1 percent, the fundamental nuclear vibration frequency is off by 4 percent. The lowest singlet state is not given as accurately. The energies and wave functions for the lowest triplet and singlet are calculated and compared for various values of internuclear separation. Discussion is made of the validity of single configuration theory, from both the Heitler-London and Hund-Mulliken points of view.