Nonadditive effects in H₂ and HF tetramers

Nonadditive three- and four-body interaction energies have been calculated for HF tetramer at the MP2/aug-cc-pVTZ level and for H₂ tetramer at the MP4(SDTQ)/aug-cc-pVTZ level using the so-called fifteen-point method. Calculated results show that with intermolecular distances decreasing from 3.0 to 1.7 Å the nonadditive three- and four-body interactions may be: (a) more and more attractive; (b) more and more repulsive; or (c) extremely weak. Strangely the minimum point of nonadditive three- and four-body interaction potentials has not been found up to now. For both H₂ and HF tetramers the nonadditive four-body interaction energy makes a negligible contribution to total binding energy when intermolecular distances are compressed from 3.0 to 1.7 Å.