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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].
         


next up previous contents index
Next: Size of the CI Up: Reducing the Size of Previous: Classification of Basis Functions
C. David Sherrill
2000-04-18