The second-order perturbative treatment of molecular electronic energies is modified in a way that takes account of near-degeneracy effects. The resulting formalism (DCPT2) is just as easy to apply as the popular second-order Moller-Plesset theory (also called second-order many-body perturbation theory) but is more accurate because it is less sensitive to near-degeneracy effects. Illustrative calculations are reported for a series of small molecules, showing significant improvement for cases with lone pairs on vicinal atoms.
Bibliographical noteFunding Information:
The authors are grateful to Ivan Rossi for providing some of the coupled-cluster calculations used for comparison and to Christopher J. Cramer for discussions. This work was supported in part by the National Science Foundation under grant No. CHE94-23927.