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...
