This approach to generating an MAO representation does have some limitations. First, it assumes that there is a 1:1 mapping between bonding and antibonding orbitals. One class of exceptions can be found in electron deficient molecules (e.g. LiH will not recover 2p-like orbitals on Li, and BH 3 will not recover a 2p z orbital on B). Such species can be said to have "virtual lone pairs", whose identification is a problem that we shall not address here.