Configuration interaction CISDTQ

Few studies have employed configuration interaction with all singles, doubles, triples, and quadruples (CISDTQ) because the number of triple and quadruple substitutions grows very rapidly with the number of electrons and basis functions (cf. Table 3). CISDTQ results are most commonly reported in benchmark full Cl studies to indicate the fraction of the basis set correlation energy recovered by triples and quadruples.17,22,39,801234,241,264 Nevertheless, the CISDTQ method has occasionally been used for benchmarking in cases where the full Cl was not technically feasible, because the CISDTQ results are expected to be very close to full Cl for small molecules. For systems with eight electrons or less at their equilibrium geometries, the CISDTQ method recovers more... 

CISD. CISD (configuration interaction, single double) are LCAO expressions that treat configuration interactions by including one or two excited states. The designations CISDT and CISDTQ expand this to three and four excitations, respectively. 

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