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

The second-order dispersion energy in the coupled-pair (coupled-cluste r doubles) approximation has been derived. The coupled-pair amplitudes are subsequently used in a perturbation theory type expression to acc ount for the effects of single and triple excitations. This approach s electively sums to infinite order important classes of intramonomer co rrelation diagrams resulting in a better theoretical description of th e dispersion interaction compared to a finite-order perturbation treat ment. Numerical results have been obtained for He-2, Ar-H-2, Ar-HF, (H F)(2), (H2O)(2), and He-F- in various geometries and basis sets to ill ustrate the performance of the nonperturbative versus perturbative tre atments of the intramonomer correlation contributions to the energy of the dispersion interaction. (C) 1995 American Institute of Physics.