MR configuration interaction

In such an approach, the correlation energy is partitioned into a complete active space self-consistent field (CASSCF) part which describes the static correlations and a MR configuration interaction part for the dynamic correlations. In the case of metals we select an active space for the CASSCF calculation including the important bands around the Fermi level. The dynamic correlations are treated on top of the CASSCF wavefunction with an approximately size-extensive MR correlation method, i.e. an MR averaged coupled pair functional (MR-ACPF). 

The calculations are not all at exactly the same bond length R. The basis set is indicated after the slash in the method. R, L, C, and T are basis sets of Slater-type functions. The aug-cc-pVDZ and aug-cc-pVTZ basis sets [360] are composed of Gaussian functions. SCF stands for self-consistent-field MC, for multiconfiguration FO, for first-order Cl, for configuration interaction MR, for multireference MPn, for nth-order Mpller-Plesset perturbation theory and SDQ, for singles, doubles, and quadruples. 

One important conclusion from the full Cl benchmark studies of Bauschlicher, Taylor, Langhoif, and others in the 1980 s is that the MR-CISD method based on CASSCF orbitals provides potential energy surfaces which accurately parallel the full Cl surfaces.14,15 234,238-240 242 254 For example, the CASSCF MR-CISD method predicts singlet-triplet energy separations in CH2 and SiH2 within 0.01 kcal mol-1 and 0.03 kcal mol-1, respectively, of the full Cl results.236,238 The best results are obtained when no threshold is used for reference selection that is, when all CSFs in the CASSCF wavefunction are used as references. This CAS-ref MR-CISD procedure is intimately related107 to second-order configuration interaction (SOCI), which distributes electrons in... 

P.G. Szalay, Towards state-specihc formulation of multireference coupled-cluster theory Coupled electron pair approximations (CEPA) leading to multireference configuration interaction (MR-CI) type equations, in R.J. Bartlett (Ed.), Modem ideas in coupled-cluster methods, World Scientific, Singapore, 1997, pp. 81-123. 

This tendency is also seen in comparing the CASPT2 and multireference configuration interaction (MR-CISD -1- Q) values for the energy difference between open-shell singlet (W) vinylnitrene and 2//-azirine. The nitrene and azirine... 

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