The time-dependent density functional theory calculations for the monomers to hexamers of benzophenone and 3-methyl-4-methoxy-4'-nitrostilbene (MMONS) nanocrystals, together with their optical absorption spectra, clearly show that the excitonic intramolecular and intermolecular charge transfer (CT) characteristics largely determine the size-related optical properties observed in organic nanoparticles. The presence of dipole-dipole interactions and hydrogen bond networks in MMONS nanocrystals causes the transition energy of the intermolecular CTs to be lower than that of the intramolecular CTs and leads to the size-related effect, whereas the lack of such intermolecular interactions in benzophenone does not. (C) 2009 Elsevier B. V. All rights reserved.