Ab initio calculation of three-body interaction in the (H-2)(3) trimer

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