To elucidate the difference in charge transfer through single- and double-strand DNAs, we performed simulations based on classical molecular dynamics (MD) and semiempirical molecular orbital (MO) methods. Stable structures of DNA strands in water were determined by MD simulations and their energy levels and distributions of frontier MOs that mediate charge transfer were analyzed by the MO calculations. The transfer integrals for a hole or an electron between the neighboring DNA bases were estimated from the energy levels of frontier MOs. We obtained the current-voltage characteristics for single- and double-strand DNAs that are qualitatively in agreement with the experimental results. (c) 2007 Elsevier B.V. All rights reserved.