Journal
MOLECULAR PHYSICS
Volume 101, Issue 13, Pages 2029-2041Publisher
TAYLOR & FRANCIS LTD
DOI: 10.1080/0026897031000109329
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A new determinant-specific, effective change ('dressing') of the norm of the multireference configuration interaction (MRCI) wavefunction is proposed in order to achieve the size-consistency of the MRCI method. The new approach provides a unifying framework for analysis of size-consistent extensions of the MRCI method that are based on the coupled pair functional (CPF) strategy and lead to simplified computations of the analytical gradients. Using the new framework, a generalized multireference full coupled pair functional (MR-FCPF) method is introduced. The MR-FCPF method may be viewed as a functional counterpart of the recently proposed generalized('full') coupled electron pair approximation (CEPA), referred to as the size-consistent self-consistent CI ((SC)(CI)-C-2) method. A straightforward extension of the MR-FCPF method leads to a pseudo-functional form of the coupled cluster (CC) type formalisms. Therefore, the new approach may be used to introduce a simple alternative to existing CC-type gradient techniques. The new procedure is formally derived and compared with similar methods from the literature. Model systems calculations (H2O, LiF, CH2+) are further used to demonstrate the effect of various approximations and to elucidate the hierarchy of functional MR-CEPA schemes.
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