The performance of energy extrapolation procedures in truncated averaged coupled-pair functionals

Summary Energy extrapolation techniques in conjunction with individual configuration selection are applied to averaged coupled-pair functional expansions. In order to test the quality of this approach, benchmark calculations have been performed for N₂, the open and ring forms of O₃, and for the ground and several excited states of CuH and PdH. Reliable energy estimates are obtained for N₂ and the two transition metal hydrides and spectroscopic properties are in close agreement with the values for the non-truncated expansions. In the case of O₃ the perturbation corrections substantially underestimate the complete singles and doubles results. These deviations cancel to a large extent, however, in the calculated isomerization energy. The accuracy of the one-particle density matrix is examined by computing dipole moments for several electronic states of CuH and PdH. Deviations are significant in some cases. For the evaluation of properties the current approach requires modifications.Dedicated to Prof. W. Kutzelnigg on the occasion of his sixtieth birthday