AN ANALYSIS OF DYNAMIC LINEAR-RESPONSE PROPERTIES AT RPA LEVEL

A separation of the dynamic linear response is developed, which distin guishes between the one- and two-electron contributions to the molecul ar response, by partitioning the RPA equation. The derivation of the p artitioning is given in both an RPA, equation of motion, type approach and using the alternative, but equivalent, density matrix method. Thr ee physically distinct contributions are obtained, called the direct, interaction, and back contributions. The direct term is composed entir ely of one-electron effects, while the interaction and back terms acco unt for the electron-interaction contributions to the response. Result s for the dynamic dipole polarizability suggest that while the one-ele ctron contribution is dominant in the zero-frequency Limit, the two-el ectron contribution becomes increasingly important as the frequency of the perturbation increases. This implies that approximation of the Li near response by only one-electron contributions is acceptable for the static case, but is less relevant for the dynamic case. The ramificat ions of this observation, for the scaling of sum-over-states-type calc ulations of large molecular systems, is briefly discussed, as is the a pplication of our partitioning method to the higher polarizabilities. (C) 1995 John Wiley & Sons, Inc.