The linked singles and doubles model: An approximate theory of electron correlation based on the coupled-cluster ansatz

From the diagrammatic construction of the full coupled-cluster theory of all single and double excitations, a linearized theory, a direct configuration interaction theory (CISD), a CEPA-like theory, and a linked singles and doubles (LSD) theory are separated. These theories are then compared with one another, with the results from full fourth-order perturbation theory, and with exact results when available. The LSD model, corresponding to the removal of unlinked terms of the CISD, and its spin adapted version, appear most accurate in Pariser–Parr–Pople studies where the exact numbers are known. Examples within the localized bond model are given indicating that this model is also the most successful of those examined in generating not only the basis set correlation, but the necessary delocalization and polarization required to correct for the zeroth-order local description.