Semiempirical Molecular-Orbital Theory of Molecular Spectra. II. Approximate Open-Shell Theory

An approximate open-shell theory, based on the CNDO method, has been developed and applied to the calculation of the excited-state molecular parameters of several small polyatomic molecules. The key feature of this theory is that it optimizes the molecular orbitals for the excited-state electron configuration. A comparison of the results of this theory with those of a virtual-orbital theory [H. W. Kroto and D. P. Santry, J. Chem. Phys. 47, 792, (1967), Paper I], which differs only in the fact that the molecular orbitals are optimized for the ground-state electron configuration, indicates that a definite improvement has been obtained. The differences between these two theories are illustrated by direct comparison of the electron densities calculated by the two theories. As is to be expected, the results show the greatest improvement for excitations between orbitals differing greatly in polarity and that the virtual-orbital theory is adequate in those cases where little change in polarity is involved.