Second order configuration interaction SOCI

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... 

Complete active space (CAS) SCF plus second-order configuration interaction (SOCI) calculations [12] and MP4 calculations [13, 14] were performed. The following table lists values for the internuclear distance r, dissociation energy D, ionization energy Ej, and spectroscopic constants for NeN ( i ) Ne( S) + N ( P)) ... 

In addition to MP2, MP3, and MP4 calculations, CCSD(T), CASSCF, FOCI (First-Order Configuration Interaction), and sometimes SOCI (Second-Order Configuration Interaction) approaches have been used to ensure the convergence of the results. The complete definitions of the variational spaces used are given in [61,62,63]. Electronically-excited states have been obtained by means of the MC/P method, recently developed in our group [64,65] it couples a variational treatment to deal with the nondynamic correlation effects and a perturbation treatment to account for the dynamic correlation effects as well as the non-dynamic effects not treated at the variational level becanse of their limited contributions to the phenomena investigated. All electronic transitions reported here are vertical transition energies. 

SOCI spin-orbit configuration interaction SOFT second-order perturbation theory SOREP spin-orbit relativistic effective potential TD-DFT time dependent density functional theory ZORA zero-order regular approximation... 

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