van der Waals interaction potentials

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Abstract
The convergence of the many-body expansion for the interaction energy of helium is examined by performing matrix Hartree-Fock calculations on various geometrical arrangements of four helium atoms. Two-, three- and four-body interaction energies are determined. Basis set superposition effects are monitored by means of the site-site function counterpoise technique. The importance of four-body terms for certain geometries is demonstrated.