Intramonomer correlation contributions to first-order exchange nonadditivity in trimers

An expression for the nonadditive first-order exchange contribution in terms of the one- and two-particle density matrices of the isolated monomers is given. This formula is used to derive the intramonomer correlation contribution to the first-order exchange nonadditivity with the density matrices of 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 only in relation to the corresponding first-order uncorrelated exchange, but also in comparison to the other three-body terms. Moreover, it is fairly anisotropic, so that it cannot be neglected in accurate calculations of nonadditive effects. (C) 2000 American Institute of Physics. [S0021-9606(00)30106-4].