Correlation energy percentage recovered

The principal deficiency of CISD is the lack of the TI term, which is the main reason for CISD not being size extensive. Furthermore, this term becomes more and more important as the number of electrons increases, and CISD therefore recovers a smaller and smaller percentage of the correlation energy as the system increases. There are various approximate corrections for this lack of size extensivity which can be added to standard CISD. The most widely known of these is the Davidson correction, sometimes denoted CISD - - Q(Davidson), where the quadruples contribution is approximated as... 

Table 2 Percentage of correlation energy recovered by various Cl excitation levels for some small molecules.

Gaussian geminals improves the percentage of the correlation energy recovered by the basis set from 73-85% to 92-98%. 

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