Table 2 demonstrates the importance of various excitation classes in obtaining CI energies. We see that singles and doubles account for 95% of the correlation energy at the equilibrium geometries of the molecules listed. We see that quadruple excitations are more important than triples, at least as far as the energy is concerned. At stretched geometries, the CISD and CISDT methods become markedly poorer, yet the CISDTQ method still recovers a very high (and nearly constant) fraction of the correlation energy, suggesting that CISDTQ should give reliable results for energy differences across potential energy surfaces for molecules of this size.
Table 2: Percentage of correlation energy recovered
by various CI excitation levels for some small molecules.