| Abstract: | A separation of the dynamic linear response is developed, which distinguishes between the one- and two-electron contributions to the molecular response, by partitioning the RPA equation. The derivation of the partitioning is given in both an RPA , equation of motion, type approach and using the alternative, but equivalent, density matrix method. Three physically distinct contributions are obtained, called the direct, interaction, and back contributions. The direct term is composed entirely of one-electron effects, while the interaction and back terms account for the electron-interaction contributions to the response. Results for the dynamic dipole polarizability suggest that while the one-electron contribution is dominant in the zero-frequency limit, the two-electron contribution becomes increasingly important as the frequency of the perturbation increases. This implies that approximation of the linear response by only one-electron contributions is acceptable for the static case, but is less relevant for the dynamic case. The ramifications of this observation, for the scaling of sum-over-states-type calculations of large molecular systems, is briefly discussed, as is the application of our partitioning method to the higher polarizabilities. © 1995 John Wiley & Sons, Inc. |
