Energy Contributions of the Various Excitation Levels

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.

	Percent Corr. Energya
Molecule	CISD	CISDT	CISDTQ
BH	94.91	n/a	99.97
H2O(Re)	94.70	95.47	99.82
H2O(1.5 Re)	89.39	91.15	99.48
H2O(2.0 Re)	80.51	83.96	98.60
NH3	94.44	95.43	99.84
HF	95.41	96.49	99.86
H7+	96.36	96.87	99.96
aData from reference [6] except for H7+ data
from reference [17].