TRIPLE EXCITATION EFFECTS IN COUPLED-CLUSTER CALCULATIONS OF FREQUENCY-DEPENDENT HYPERPOLARIZABILITIES

We describe an implementation of an analytic scheme for the calculatio n of static and dynamical first hyperpolarizabilities at the CC3 level in the framework of coupled-cluster response theory. Calculations are reported for the static, the second harmonic generation, and the opti cal rectification hyperpolarizabilities of FH. The results indicate th e importance of triple excitation effects for accurate theoretical pre dictions of hyperpolarizabilities and lend further support to previous contentions that an experimental value for the second harmonic genera tion hyperpolarizabilities of FH should be reconsidered. (C) 1998 Else vier Science B.V. All rights reserved.