Unlinked cluster effects in molecular electronic structure. II. Pair correlations in the molecules HCN and HNC

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Abstract
Valence‐shell correlation energies for the HCN and HNC molecules are analyzed into pair contributions at the independent‐pair, CI, and coupled‐pair level. The correlation effects for each system are very similar, and are easily related to the spatial extension and interpenetration of the localized SCF orbitals which form the reference configuration. In addition to the recently proposed ’’coupled‐pair approximation’’ (CPA), two approximate methods, including the widely‐used CEPA technique, are used, and the results are compared with those obtained using the full treatment. The CPA″ method proposed earlier is found to give energy results in agreement with the full CPA to ’’chemical accuracy’’ (1kcal/mole); the agreement obtained with CEPA is not as good, although there is still a substantial improvement over the CI values.