Energy extrapolation in CI calculations

A method of extrapolating the CI energy of large configuration subspaces is discussed and illustrated in a series of examples. Secular equations for selected groups of configurations are solved, while the influence of the remaining elements in the subspace is accounted for in a statistical manner. Criteria for judging the reliability of the extrapolation technique are developed and evidence is presented underscoring the need to go beyond the level of truncated CI calculations in obtaining excitation energy and potential surface results.

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