NATURAL ENERGY DECOMPOSITION ANALYSIS - AN ENERGY PARTITIONING PROCEDURE FOR MOLECULAR-INTERACTIONS WITH APPLICATION TO WEAK HYDROGEN-BONDING, STRONG IONIC, AND MODERATE DONOR-ACCEPTOR INTERACTIONS

We present a procedure for partitioning the Hartree-Fock self-consiste nt-field (SCF) interaction energy into electrostatic, charge transfer, and deformation components. The natural bond orbital (NBO) approach o f Weinhold and co-workers is employed to construct intermediate superm olecule and fragment wave functions that satisfy the Pauli exclusion p rinciple, thereby avoiding the principal deficiency of the popular Kit aura-Morokuma energy decomposition scheme. The function counterpoise m ethod of Boys and Bernardi enters the procedure naturally, providing a n estimate of basis set superposition error (BSSE). We find that the e nergy components exhibit little basis set dependence when BSSE is smal l. Applications are presented for several representative molecular and ion complexes: the weak hydrogen bond of the water dimer, the strong ionic interaction of the alkali metal hydrides, and the moderate donor -acceptor interactions of BH3NH3 and BH3CO. Electrostatic interaction dominates the long-range region of the potential energy surface and ch arge transfer is strongly stabilizing for fragments within van der Waa ls contact. The repulsive interaction in the short range region of the potential arises from deformation as the fragment wave functions dist ort to avoid significant interpenetration.