Direct calculation of non-adiabatic states using non-orthogonal CI. Application to the study of the (ArCO)+ cluster Original Research Article

We show that it is possible to describe the (ArCO)+ system with reasonable 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 correction of the asymptotic error by an empirical shift of the non-adiabatic energies straightforward. We discuss the value of the binding energy D0 of the cluster. We use the linear ArCO geometry and the Σ symmetry. The fast (preoptimised) version of the method yields D0=0.56±0.04 eV, in qualitative agreement with the experimental value of Ng (0.7±0.06 eV). The full optimisation of the orbitals increases the D0 value by 0.12 eV. The equilibrium geometry of the cluster is RArC=4.7 au. The polarisation contribution is 40% of the binding energy.