On the many-body contributions to the interaction polarisability and hyperpolarisability of Hen

The many-body contributions to the interaction polarisability and hyperpolarisability of He_{n (n}=3,4, 5), for various configurations and distances have been determined and analysed. Several cases have been found where the three-body terms contribute more than 20% to or of He_n. The remarkable dependence of the above interaction properties on the internuclear distances and the shape of the cluster has been demonstrated. The interaction hyperpolarisabilities are shown to be uniquely sensitive probes of the electronic structure changes induced by variation of the cluster configuration. The results were computed by employing a computational procedure which relies on anab initio wave function, McWeeny's et al. coupled Hartree-Fock perturbation theory and an efficient algorithm for the determination of hyperpolarisabilities starting from a non-orthogonal basis set. The function counterpoise method has been used to reduce the basis set superposition error.