2ND-ORDER POLARIZATION PROPAGATOR APPROXIMATION WITH COUPLED-CLUSTER SINGLES AND DOUBLES AMPLITUDES - SOPPA(CCSD) - THE POLARIZABILITY AND HYPERPOLARIZABILITY OF LI-

A new modification of the second-order polarization propagator approxi mation (SOPPA) is presented. In the new method, called SOPPA(CCSD), al l first-order Moller-Plesset doubles and second-order Moller-Plesset s ingles correlation coefficients are replaced with the corresponding co upled-cluster singles and doubles (CCSD) T-2 and T-1 amplitudes. This is in contrast to the earlier coupled-cluster polarization propagator approximation (CCSDPPA) in which only one of the doubles correlation c oefficients was replaced in the unrelaxed second-order density matrix. The importance of this modification is investigated for the dipole po larizability, dipole oscillator strength sum rules and second hyperpol arizability of Li-, which is known to be a difficult case for perturba tive methods. The SOPPA(CCSD) values are compared with previous and ne w full CI results for the same basis sets.