Atomic correlation energies. IV. Perturbations and near degeneracy

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Abstract
For. pt.III see ibid., vol.11, p.2969 (1978). The perturbative calculation of correlation energies in atoms with near-degenerate ground states is considered. The authors observe that a small CI calculation in the near-degenerate space allows them to evaluate the most important terms beyond E(2). By adding these higher-order corrections to E(2) a better approximation to the correlation energies is obtained. Applying this procedure to Be, B and C they obtain 103.8%, 100.9% and 98.8% of the empirical correlation energy, compared to 80%, 89.2% and 94.3% in second order.