SIZE-CONSISTENT SELECTED CONFIGURATION-INTERACTION CALCULATIONS - A FEW TESTS OF EFFICIENCY

As recently shown, size extensivity of any truncated or selected CI ma y be obtained by a self-consistent dressing of the determinant energie s. This dressing has been implemented in two direct CI algorithms and its efficiency is illustrated on a series of eight-electron problems f or which full-CI results are known (HF, H2O, NH3 in DZP basis sets). T he dressing has been applied to CIs defined by single and double excit ations on complete active spaces, single and double substitutions on s elected reference spaces or by iterative double threshold selection. I n most cases the dressing reduces the error by at least a factor of tw o to three and in selected CIs it makes it possible to save about one order of magnitude in the size of the CI. The best results are obtaine d from iteratively selected CIs, the error being lower than 5 x 10(-4) hartree for 2 x 10(5) determinant CIs.