The lowest-energy bonding and antibonding molecular orbitals (MO) for a homonuclear diatomic molecule are shown, as the internuclear distance
is varied. These MOs are designated 1
(or 1s
) and 1
(or 1s
), respectively. The red contours designate negative values of the wavefunction. In the separated atom limit (large
), the MOs approach a pair of noninteracting 1
s atomic orbitals. In the united atom limit (
), the 1
and 1
collapse to 1
s and 2
atomic orbitals, respectively. At the equilibrium internuclear distance, when the red dot is at the minimum of the energy curve, the bonding orbital is optimally effective.
