Beryllium clusters calculations

The physical picture is opposite to the beryllium cluster case. The 2-body interaction energies are large and stabilizing while the 3-body forces play a destabilizing role. The attractive 4-body forces are decisive for formation the Li4 cluster since the 2-body attraction in the Li4 is smaller than the 3-body repulsion. It means that all terms in the m-body decomposition of the Li4 interaction energy are important. The same was demonstrated in the accurate calculation of the He4 and Nc4 clusters by Wells and Wilson ". ... 

The lowest, virtual, canonical HF orbitals of the neutrals consist chiefly of valence s and p basis functions. Singly occupied spin orbitals that occur only in UHF calculations on the anion are composed of the same AOs. Both types of these one-electron wave funetions may be considered approximations to the Dyson orbitals [85,86] corresponding to the EAs of the beryllium clusters. In the framework of the electron propagator theory [85], the Dyson orbitals are overlaps between an N-electron reference state and final states with N electrons. They form an overcomplete set and are not necessarily normalized to unity. 

The CC method (with the cluster operator truncated at double excitations) gives energy EseBe for two beryllium atoms at very large internuclear distance R, and the energy E e for a single beryllium atom. In the calculations using this method ... 

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