Size-extensive quadratic CI methods including quadruple excitations: QCISDTQ and QCISDTQ(6) - On the importance of four-electron correlation effects

Size-extensive Quadratic CI with single (S), double (D), triple (T), and qu adruple (Q) excitations (QCISDTQ) and its non-iterative extension to a meth od correct at sixth-order perturbation theory, QCISDTQ(6), are developed an d applied to electronic systems, for which full CI (FCI) results are availa ble. It is shown that QCISDTQ results are more accurate than either QCISDT or CCSDT. In particular, QCISDTQ(6) correlation energies differ from FCI co rrelation energies on the average by just 0.04 mhartree. The improvement is caused preferentially by the balanced addition of connected and disconnect ed four-electron correlation effects, which confirms observations that four -electron correlation effects are important for a balanced description of e lectron correlation. (C) 2000 Elsevier Science B.V. All rights reserved.