A combination of the reference interaction-site model theory and site-site Smoluchowski-Vlasov equation is applied to estimate the dynamic response function of the average-energy relaxation of the solute-solvent system, S-S(t). We calculate S-S(t) for 13 model solutes with different structure, from a simple ion to an octopole, in a polar solvent. The partial charges of the ions and multipoles are changed to investigate nonlinear character of S-S(t). The "nonlinear character" we study here corresponds to the response of the solvent fluctuation after the sudden change of the solute charge-distribution. Our present results reveal that S-S(t) depends on the molecular structure and charge distribution of the solute. S-S(t) is decomposed into two parts: one corresponding to the optical mode of solvent, the other to the acoustic mode. We show that for multipoles the optical mode is responsible for the fast part of S-S(t), while the acoustic mode plays an important role in the slower dynamics. The dual nature of S-S(t) is essential for the nonlinearity of solvation dynamics. (C) 2003 American Institute of Physics.