DISPERSION ENERGY IN THE COUPLED PAIR APPROXIMATION WITH NONITERATIVE INCLUSION OF SINGLE AND TRIPLE EXCITATIONS

The second-order dispersion energy in the coupled-pair (coupled-cluster doubles) approximation has been derived. The coupled-pair amplitudes are subsequently used in a perturbation theory type expression to account for the effects of single and triple excitations. This approach selectively sums to infinite order important classes of intramonomer correlation diagrams resulting in a better theoretical description of the dispersion interaction compared to a finite-order perturbation treatment. Numerical results have been obtained for He-2, Ar-H-2, Ar-HF, (HF)(2), (H2O)(2), and He-F- in various geometries and basis sets to illustrate the performance of the nonperturbative versus perturbative treatments of the intramonomer correlation contributions to the energy of the dispersion interaction. (C) 1995 American Institute of Physics.