T-5 operator in coupled cluster calculations

The first attempt to include the connected pentuple excitation, T-5, into t he coupled cluster method is reported. Construction of the lowest order, i. e. fourth-order T-5 operator requires an inclusion of eight diagrammatic te rms into the T-5 equation. The method is applied to several small systems f or which a comparison with full configuration interaction results can be ma de. (C) 2000 Elsevier Science B.V. All rights reserved.