We will refer to these virtual orbitals as "second order antibonding" (AB2) MOs to emphasize their second order origins, and their 1:1 correspondence with bonding MOs. In terms of existing literature, the AB2s are directly related to the "orbital-specific virtual" (OSV) orbitals that are sometimes used to evaluate the correlation energy. Each AB2 orbital is the most important OSV for a given localized bonding orbital. Of course the reason for selecting the amplitudes associated with MP2 is computational efficiency. The exact limit of this procedure would be to diagonalize the corresponding exact (i.e. from Full CI) doubles amplitudes; T i ab , via Eq. 9.