Natural bond orbital analysis: a critical overview of relationships to alternative bonding perspectives.

We sketch the basic principles of natural bond orbital (NBO) theory, including critical discussion of its relationship to alternative bonding concepts and selected illustrations of its application to a broad spectrum of chemical bonding motifs. Particular emphasis is placed on the close NBO connections to prequantal bonding, and electromerism concepts, as well as the deep roots in quantal eigenvalue, superposition, and Pauli exclusion concepts that are manifested in many aspects of NBO donor-acceptor analysis. With respect to leading alternative perspectives, we identify similarities and differences that distinguish NBO theory from the corresponding precepts of valence bond theory, molecular orbital theory, and Bader's quantum theory of atoms in molecules, with critical discussion of the assumptions underlying characteristic differences in each case.