In polyatomic molecules it is usual to recognize physically distinct groups of electrons (e.g., inner shells, σ bonds, π electrons). The π electrons of conjugated molecules have received most attention, other groups of electrons being recognized only as a ``framework'' with empirically determined characteristics. In this series of papers, all groups of electrons are admitted, each group (localized or nonlocalized) being made self-consistent in the field of the others. In this way bond properties, π-electron parameters, etc., may be determined from first principles, taking into account the whole molecular environment. Excited states may be determined by admitting ``excitons'' (i.e., localized excitations) within the various groups. The theory is illustrated by means of a completely nonempirical calculation on the methane molecule. The ground state is somewhat better than that given by approximate SCF theory, the excited states are somewhat worse. One advantage of the approach is that it lends itself admirably to semiempirical development.

Klessinger, M., & McWeeny, R. (1965). Self-Consistent Group Calculations on Polyatomic Molecules. I. Basic Theory with an Application to Methane. The Journal of Chemical Physics, 42(10), 3343–3354.