A b i n i t i o study of the nonadditive effects in the trimer of ammonia

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Abstract
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.