DIRECT CALCULATION OF NONADIABATIC STATES USING NONORTHOGONAL CI - APPLICATION TO THE STUDY OF THE (ARCO)(+) CLUSTER

We show that it is possible to describe the (ArCO)(+) system with reas onable accuracy, despite the use of a poor description (SCF or valence CAS) of the separate units. We use a non-orthogonal CI technique for building the two states Ar-CO+ and Ar+-CO separately. This makes the c orrection of the asymptotic error by an empirical shift of the non-adi abatic energies straightforward. We discuss the value of the binding e nergy D-0 of the cluster. We use the linear Ar-CO geometry and the Sig ma symmetry. The fast (preoptimised) version of the method yields D-0 = 0.56 +/- 0.04 eV, in qualitative agreement with the experimental val ue of Ng (0.7 +/- 0.06 eV). The full optimisation of the orbitals incr eases the D, value by 0.12 eV. The equilibrium geometry of the cluster is R(ArC) = 4.7 au. The polarisation contribution is 40% of the bindi ng energy.