An environmental approach to molecular interactions: implementation within MOLPRO

We present the implementation into the MOLPRO package of a model for the interaction of a central system with its surrounding environment. The properties of a target system enclosed by a non−covalently bound environment or solvent are modelled as those of a system embedded into the effective pseudopotential arising from the exact electrostatic coulomb potential and the approximated exchange−repulsion potential. For the latter we use the charge−density overlap model which relates the exchange−repulsion interaction energy between two species with the overlap of their ground−state electron charge densities. The solutions of the modified Hartree−Fock equations for the target system are obtained self−consistently. This way the exchange−induction effects arising from the converged electron−charge density of the embedded system are implicitly covered. Further inclusion of the correlation effects is provided by the use of post−Hartree−Fock and density−functional techniques available in the MOLPRO package. The computational and conceptual advantages provided by such approach are shown in the calculation of the dipole polarisabilities of halide and chalcogenide compounds in different environments.