Truncated Quadratic Configuration Interaction Methods

The L7 database was developed on the basis of binding energies (E ) of seven mostly dispersion-stabihzed intermolecular complexes [6]. QCISD(T)/CBS was adopted to compute because of an advantage in efficiency over CCSD(T) in the utilized PQS program package [7]. QCISD(T) is believed to have an accuracy similar to that of CCSD(T) [8]. On the basis of L7, the performance of several commonly used wave function, DFT, and semiempirical quantum chemical methods was evaluated on large intermolecular complexes. 

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There is also a hierarchy of electron correlation procedures. The Hartree-Fock (HF) approximation neglects correlation of electrons with antiparallel spins. Increasing levels of accuracy of electron correlation treatment are achieved by Mpller-Plesset perturbation theory truncated at the second (MP2), third (MP3), or fourth (MP4) order. Further inclusion of electron correlation is achieved by methods such as quadratic configuration interaction with single, double, and (perturbatively calculated) triple excitations [QCISD(T)], and by the analogous coupled cluster theory [CCSD(T)] [8],... 

For correlated methods such as truncated configuration interaction (CID or CISD), coupled cluster (CCD or CCSD), quadratic configuration interaction (QCISD) and Brueckner doubles (BD) (see Configuration Interaction and Coupled-cbister Theory), the energy and wavefunction can be written as... 

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