Size-extensive quadratic CI methods including quadruple ž / excitations: QCISDTQ and QCISDTQ 6 – On the importance of four-electron correlation effects
نویسندگان
چکیده
Ž . Ž . Ž . Ž . Ž . Size-extensive Quadratic CI with single S , double D , triple T , and quadruple Q excitations QCISDTQ and its Ž . non-iterative extension to a method correct at sixth-order perturbation theory, QCISDTQ 6 , are developed and applied to Ž . electronic systems, for which full CI FCI results are available. It is shown that QCISDTQ results are more accurate than Ž . either QCISDT or CCSDT. In particular, QCISDTQ 6 correlation energies differ from FCI correlation energies on the average by just 0.04 mhartree. The improvement is caused preferentially by the balanced addition of connected and disconnected four-electron correlation effects, which confirms observations that four-electron correlation effects are important for a balanced description of electron correlation. q 2000 Elsevier Science B.V. All rights reserved.
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