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Quadratic configuration interaction

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Title: Quadratic configuration interaction  
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Subject: QCI, Post-Hartree–Fock, Index of physics articles (Q), PQS (chemical), Quantum chemistry composite methods
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Quadratic configuration interaction

Quadratic configuration interaction[1] (QCI) is an extension of configuration interaction[2] that corrects for size-consistency errors in single and double excitation CI methods (CISD).[3]

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.[4] QCISD can be improved by the same perturbative inclusion of unlinked triples to give QCISD(T). This gives similar results to CCSD(T).[5]


  1. ^ John A. Pople, Martin Head‐Gordon, and Krishnan Raghavachari (1987). "Quadratic configuration interaction. A general technique for determining electron correlation energies". The Journal of Chemical Physics (American Institute of Physics) 87 (10): 5968–35975.  
  2. ^ David Maurice and Martin Head-Gordon (May 10, 1999). "Analytical second derivatives for excited electronic states using the single excitation configuration interaction method: theory and application to benzo[a]pyrene and chalcone". Molecular Physics (Taylor & Francis) 96 (10): 1533–1541.  
  3. ^ Martin Head-Gordon, Rudolph J. Rico, Manabu Oumi, and Timothy J. Lee (1994). "A doubles correction to electronic excited states from configuration interaction in the space of single substitutions". Chemical Physics Letters (Elsevier) 219 (1–2): 21–29.  
  4. ^ George D. Purvis and Rodney J. Bartlett (1982). "A full coupled‐cluster singles and doubles model: The inclusion of disconnected triples". The Journal of Chemical Physics (The American Institute of Physics) 76 (4): 1910–1919.  
  5. ^ Krishnan Raghavachari, Gary W. Trucks, John A. Pople and, Martin Head-Gordon (March 24, 1989). "A fifth-order perturbation comparison of electron correlation theories". Chemical Physics Letters (Elsevier Science) 157 (6): 479–483.  

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