DZP H4 model

Table II. The SR, 2R, 4R and (2/2)R CISD ground state energies together with various Davidson-type corrections relative to the FCI energy (in mH) for the DZP H4 model with 0 < a < 0.5. The nonparallelism errors (NPE) are givenin the last two rows. See the text for details. 

We have also seen that the said complementarity of the CC and Cl approaches can be exploited in a reversed order, namely that we can rely on the MR GISD results and adjust them for their lack of dynamical correlation via the Davidson-like corrections that are based on the CC theory, as proposed by Meissner et al. [10,72-74]. We have seen that, at least for the studied DZP H4 model, either variant leads to excellent results. 

Fig. 22.2. Plot of energy difference [A (mH) = (fipQ — iVethod)] of Ih ground state of H4 model using DZP basis.

TABLE VI. Correlation energies for the first biexcited state of the H4 DZP model obtained using various multireference correlation techniques within a two-dimensional model space spanned by the ground state configuration 1122) cind the first biexcited configuration 1133). For simplicity, we present only differences from corresponding FCI values (A = X - FCI) in mHartree. 

Comparison of correlation energies (all signs reversed) for the ground state of the H4 DZP model (in mhartree), using a (2,2) model space. The AL-RMR-CCSD-i methods are simply designated as AL-RMR-i. 

The reduction in the computational effort is most pronounced when only a small subset of possible tj° -amplitudes is employed. We consider two thresholds for the tj° -amplitudes, viz., 0.02 and 0.1, which means that only the amplitudes that are greater than 0.02 and 0.1, respectively, are taken into account. When applying these criteria to excitations producing multideter-minantal configuration state functions, all the amplitudes associated with the excitations from the reference to these determinants are accounted for. The subscript at the energies presented in Tables 1-5 indicates the number of nonzero tj° -amplitudes, e.g., in Table 1 for the H4 DZP model, we use 1... 

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