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

As recently shown, size extensivity of any truncated or selected CI may be obtained by a self-consistent dressing of the determinant energies. This dressing has been implemented in two direct CI algorithms and its efficiency is illustrated on a series of eight-electron problems for which full-CI results are known (HF, H2O, NH3 in DZP basis sets). The dressing has been applied to CIs defined by single and double excitations on complete active spaces, single and double substitutions on selected reference spaces or by iterative double threshold selection. In most cases the dressing reduces the error by at least a factor of two 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 obtained from iteratively selected CIs, the error being lower than 5 x 10(-4) hartree for 2 x 10(5) determinant CIs.