COMPARISON OF MOROKUMA AND PERTURBATION-THEORY APPROACHES TO DECOMPOSITION OF INTERACTION ENERGY - (NH4)+ ... NH3

The interaction energy between NH4 + and NH3 is decomposed for a range of intermolecular separations within the context of a variety of basis sets including 6-31G**, 6-31+G**, and Sadlej polarization sets of type [5s3p2d/3s2p], including a set of f-functions as well. The electrostatic component dominates the interaction in all cases. At separations near the minimum or closer, the Morokuma second-order components (POL and CT) blow up. In contrast, ΔESCF def, representing mutual polarization and orbital relaxation, is in good agreement with the induction energy computed via perturbation theory. Correction of the superposition error is noted to be important for this agreement and, indeed, for all components. In fact, following this correction, even the smaller sets provide accurate measures of the exchange term. The second-order correlation correction to the electrostatic interaction ε{lunate}es 12 is computed and found to be fairly small. The dispersion energy embodied in ε{lunate}disp 20 is considerably larger than the latter term and grows in proportion to the flexibility of the basis set. © 1990.