AB-INITIO CALCULATION OF 3-BODY INTERACTION IN THE (H-2)(3) TRIMER

The non-additivity in the (H-2)(3) trimer is calculated at different g eometries using an extended geminal model. The isotropic part of the t hree-body non-additive potential energy is derived. It is shown that t he three-body potential is converging relatively rapidly with respect to the size of the basis set. With a 6s,3p,2d basis, near the equilibr ium geometry, the global error of the three-body potential is estimate d to be of only 7%. At large distances the results are in accordance w ith the Axilrod-Teller-Muto triple-dipole approximation.