Convergence of the ab initio many-body expansion for the cohesive energy of solid mercury

A many-body expansion for mercury clusters of the form E=∑_iγ_i+∑_i<jΔγ_ij+∑_i<j<kΔγ_ijk+…, does not converge smoothly with increasing cluster size towards the solid state. Even for smaller cluster sizes (up to n=6 ), where van der Waals forces still dominate, one observes bad convergence behavior. For solid mercury the convergence of the many-body expansion can dramatically be improved by an incremental procedure within an embedded cluster approach. Here one adds the coupled cluster many-body electron correlation contributions of the embedded cluster to the bulk HF energy. In this way we obtain a cohesive energy (not corrected for zero-point vibration) of 0.79eV in very good agreement with the experimental value of 0.79eV .