CALCULATION OF SIZE-INTENSIVE TRANSITION MOMENTS FROM THE COUPLED CLUSTER SINGLES AND DOUBLES LINEAR-RESPONSE FUNCTION

Coupled cluster singles and doubles linear response (CCLR) calculation s have been carried out for excitation energies and dipole transition strengths for the lowest excitations-in LiH, CH+, and C-4 and the resu lts compared with the results from a CI-like approach to equation of m otion coupled cluster (EOMCC). The transition strengths are similar in the two approaches for single molecule calculations on small systems. However, the CCLR approach gives size-intensive dipole transition str engths, while the EOMCC formalism does not. Thus, EOMCC calculations c an give unphysically dipole transition strengths, e.g., in EOMCC calcu lations on a sequence of noninteracting LiH systems we obtained a nega tive dipole strength for the lowest totally symmetric dipole allowed t ransition for 19 or more noninteracting LiH systems. The CCLR approach is shown to be a very attractive ''black box'' approach for the calcu lation of transition moments.