Efficient evaluation of poly-electron populations in natural bond orbital analysis

We show how a simple natural orbital-based modification of Karafiloglou's poly-electron population analysis (PEPA) [generalizing conventional one-electron natural population analysis (NPA)] allows efficient numerical evaluation of Born probabilities (populations) for an unlimited variety of localized electronic excitation patterns. The computational advantages are illustrated by simple numerical applications to CH3NH2 and benzene in the framework of natural bond orbital (NBO)-based NPEPA keyword implementation in the forthcoming NBO 7.0 program.