We investigate the longitudinal static second hyperpolarizability (gamma) of open-shell singlet diphenalenyl radical dimer using the spin-unrestricted density functional theory method. This dimer is known to be a model unit of a slipped stacked linear chain with an unusually short average pi-pi distance of 3.137 angstrom. The longitudinal gamma per monomer for this dimer is found to be significantly (about twice) enhanced as compared to that of monomer. This originates in the field induced intermolecular virtual charge transfer between both-end phenalenyl rings through the strong covalent interaction between the unpaired electrons of the cofacial phenalenyl rings. (c) 2008 Elsevier B. V. All rights reserved.