Externally corrected linear coupled cluster approach (ecLCCSD): application to excited states

The linear version of the externally corrected coupled cluster method with singles and doubles (ecLCCSD) has proved to be a highly accurate approximat ion for the calculation of the ground state energy in presence of quasidege neracy. Since this method is derived within the single reference SR-CC form ulation, its application to excited states is less obvious (although possib le) but can be justified. We explore in this paper the performance of this approach describing excited states that are dominated by a single closed-sh ell determinant. (C) 2001 Elsevier Science B.V. All rights reserved.