Deadwood in configuration spaces. II. Singles plus doubles and singles plus doubles plus triples plus quadruples spaces

The effect of truncating singles+doubles configuration interaction (CISD) and singles + doubles + triples + quadruples (CISDTQ) spaces on the energies of the systems Ne, H2O, CO and C-2 is investigated through the use of a previously described, general, selected CI program. CI expansions generated by Hartree-Fock orbitals as well as by natural orbitals are examined and the latter typically exhibit faster convergence as regards the energy. For the CISD wavefunctions of Ne, H2O, CO and C-2, chemical accuracy is reached by using, respectively, 34, 47, 53 and 55% of the full sets. For the triples + quadruples parts of the wavefunctions on the other hand, chemical accuracy is already reached by using only 1, 4, 6 and 9% of the respective full sets.