natural atomic orbital (NAO)
-shell atomic orbital whose derivation involves diagonalising the localized block
of the full density matrix of a given molecule associated with basis functions
on that atom. A distinguishing feature of NAO
s is that they meet the simultaneous requirement of orthonormality and maximum occupancy. For isolated atoms, NAO
s coincide with natural orbitals. In a polyatomic molecule the NAO
s (in contrast to natural orbitals that become delocalised over all nuclear centres) mostly retain one-centre character, and thus are optimal for describing the molecular electron density
around each atomic centre.