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].