Abstract
Electron correlation energies are calculated for a series of small molecules by fourth-order Moller-Plesset perturbation theory (MP4) and by quadratic configuration interaction methods. A method is suggested for partitioning the correlation contributions beyond fourth order that result from single, double, and triple substitutions. Detailed comparison of the results reveals that termination of the MP perturbation series at fourth order is inadequate for some electron-deficient species (inadequate treatment of double substitutions), for triply-bonded molecules (overestimation of the triples effect), and for some unsaturated radicals (poor treatment of single substitutions).
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