A 2ND-ORDER DOUBLES CORRECTION TO EXCITATION-ENERGIES IN THE RANDOM-PHASE-APPROXIMATION

A computationally tractable doubles correction to excitation energies obtained in the random-phase approximation (RPA(D)) is presented. It g ives excitation energies correct through second order in the ground-st ate fluctuation potential. RPA(D) has been tested for H2O, N-2, Ne, CH 2, BH and C-2 using full configuration interaction (FCI) results as be nchmarks. Comparison is made with other existing models of similar com plexity. Overall the accuracy of excitation energies calculated using RPA(D) is similar to that obtained in the CC2, CIS(D) and SOPPA method s. The errors of the random-phase approximation excitation energies ar e typically reduced by a factor of two using the RPA(D) correction. Th e mean absolute deviation of RPA(D) and FCI excitation energies is 0.5 1 eV and for the majority of the excitations the RPA(D) results are lo wer than the FCI results. (C) 1998 Published by Elsevier Science B.V.