ABINITIO STUDY OF THE NONADDITIVE EFFECTS IN THE TRIMER OF AMMONIA

The three-body potential in the trimer of ammonia is analyzed in terms of Heitler-London (HL)-exchange, self-consistent field (SCF)-deformation, induction, and dispersion nonadditivities. The nonadditive term is due largely to the SCF-deformation effect. However, its anisotropy resembles more that of the HL-exchange nonadditivity. Correlation effects do not contribute significantly to the nonadditivity. The trimer is of C3h symmetry. The geometry is determined at the level of pairwise interactions. Apart from a slight shortening of the N-N distance, the three-body effect has virtually no influence on the mutual orientations of subsystems in the trimer. Nonadditive properties of ammonia are compared to those of other trimers: (HF)3; (HCl)3; (H2O)3; and (CH4)3. An examination of the basis set dependence of the components of three-body effect leads us to believe that the present treatment yields three-body potential with accuracy better than 0.1 kcal/mol for the trimer of ammonia, as well as for other polar trimers.