Problem of ``Vertical'' Correlation in Pi-Electron Systems

Abstract
The problem of ``vertical'' correlation is re‐examined. Defined in terms of the two‐electron probability distribution function, ``vertical'' correlation is separated from the related ``in—out'' correlation effect. A study is made of the planar methyl anion and its isoelectronic analog, the planar ammonia molecule, to assess the importance of sigma antibonding orbitals for the correlation of the 2pz2 pair. These contribute −0.092 eV for planar CH3 and −0.120 eV for planar NH3. The implications for Σ—II separability are discussed.