Intramonomer correlation contributions to first-order exchange nonadditivity in trimers

An expression for the nonadditive first-order exchange contribution in term s of the one- and two-particle density matrices of the isolated monomers is given. This formula is used to derive the intramonomer correlation contrib ution to the first-order exchange nonadditivity with the density matrices o f the isolated monomers computed at the second-order Moller-Plesset level. As an illustrative example of the theory the trimer Ar 2-HF is considered. It is shown that the correlation contribution is far from negligible, not o nly in relation to the corresponding first-order uncorrelated exchange, but also in comparison to the other three-body terms. Moreover, it is fairly a nisotropic, so that it cannot be neglected in accurate calculations of nona dditive effects. (C) 2000 American Institute of Physics. [S0021-9606(00)301 06-4].