Ground-state correlations and linear response of metal clusters

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Abstract
The composition of the random-phase-approximation ground state in metal clusters is explicitly determined. It is shown that, due to the long-range character of the Coulomb interaction, multiconfigurational, higher-order correlations in the ground state play a more important role than had been previously assumed. However, the deviations between single-particle occupation probabilities associated with the correlated ground state and the uncorrelated reference determinant are found to be small, thus validating the quasiboson approximation. The reliability of various many-body approaches in describing excited collective modes depends crucially on their ability of adequately approximating these correlations in the composition of the ground-state wave function. In this context, the consistency of random-phase-approximation and configuration-interaction results in metal clusters is assessed through a comparison with established experimental trends and known theoretical limits of the exact many-body problem.