quadratic configuration interaction

Quadratic configuration interaction{{cite journal | title = Quadratic configuration interaction. A general technique for determining electron correlation energies |author1=John A. Pople |author2=Martin Head‐Gordon |author3=Krishnan Raghavachari | journal = The Journal of Chemical Physics | volume = 87 |issue =10 | pages = 5968–35975 | publisher = American Institute of Physics | year = 1987 | doi = 10.1063/1.453520 |bibcode = 1987JChPh..87.5968P }} (QCI) is an extension of configuration interaction{{cite journal | title = Analytical second derivatives for excited electronic states using the single excitation configuration interaction method: theory and application to benzo[a]pyrene and chalcone | journal = Molecular Physics | volume = 96 |issue =10 | pages = 1533–1541 | publisher = Taylor & Francis | date = May 10, 1999 | doi = 10.1080/00268979909483096 |author1=David Maurice |author2=Martin Head-Gordon|bibcode = 1999MolPh..96.1533M }} that corrects for size-consistency errors in single and double excitation CI methods (CISD).{{cite journal | title = A doubles correction to electronic excited states from configuration interaction in the space of single substitutions | journal = Chemical Physics Letters | volume = 219 |issue =1–2 | pages = 21–29 | publisher = Elsevier | year = 1994 | doi = 10.1016/0009-2614(94)00070-0 |author1=Martin Head-Gordon |author2=Rudolph J. Rico |author3=Manabu Oumi |author4=Timothy J. Lee |bibcode = 1994CPL...219...21H | url = https://zenodo.org/record/1253844 }}

Size-consistency means that the energy of two non-interacting (i.e. at large distance apart) molecules calculated directly will be the sum of the energies of the two molecules calculated separately. This method called QCISD was developed in the group of John Pople. It gives results that are comparable to the coupled cluster method, CCSD.{{cite journal | title = A full coupled‐cluster singles and doubles model: The inclusion of disconnected triples | journal = The Journal of Chemical Physics | volume = 76 |issue =4 | pages = 1910–1919 | publisher = The American Institute of Physics | year = 1982 | doi = 10.1063/1.443164 |author1=George D. Purvis |author2=Rodney J. Bartlett|bibcode = 1982JChPh..76.1910P }} QCISD can be improved by the same perturbative inclusion of unlinked triples to give QCISD(T). This gives similar results to CCSD(T).{{cite journal | title = A fifth-order perturbation comparison of electron correlation theories | journal = Chemical Physics Letters | volume = 157 |issue =6 | pages = 479–483 | publisher = Elsevier Science | date = March 24, 1989 | doi = 10.1016/S0009-2614(89)87395-6 | author = Krishnan Raghavachari, Gary W. Trucks, John A. Pople and, Martin Head-Gordon|bibcode = 1989CPL...157..479R }}