Energy gap, Mulliken population, and bond-overlap population analyses were carried out to investigate the intermolecular interaction modulation in pentacene clusters by neighboring artificially charged pentacene molecules, which model a charge-carrier injection into a crystal. Two common pentacene clusters, type-1 and type-2, obtained from the crystal structure of pentacene, were examined. A larger decrease in the band gap was observed for the type-2 orientation, whereas a positive intermolecular interaction was observed for the type-1 cluster. Both closing of the energy gap and the positive intermolecular interaction (bonding nature) were found only in the extreme n-type state. (C) 2013 Elsevier B.V. All rights reserved.