Inclusion of ion-pair states in the diatomics-in-molecules description of potential energy surfaces: van der Waals complexes of He-Cl-2 and Ar-Cl-2

It is shown that the inclusion of excited ionic configurations in the diatomics-in-molecules (DIM) Hamiltonian serves as a natural means to account for main features of non-additivity in three-body potential energy surfaces of He-Cl-2 and Ar-Cl-2 van der Waals complexes. For ground state Cl-2((1) Sigma(g)), while consideration of only neutral configurations leads to T-shaped isomers, inclusion of the excited Cl+Cl- configuration stabilizes the linear isomer and destabilizes the T-shaped isomer. Within the same formalism, the excited Cl-2((3) Pi) only sustains minima in the T-shaped isomer. Potential energy surfaces created with a minimal DIM basis are constructed and shown to compare favorably with the most accurate ab initio surfaces and experiments. Analytical forms are given for the three-body surfaces, meant for fitting purposes and as a convenience in simulations of dynamics. (C) 1997 Elsevier Science B.V.