Molecular wire electronic state crossing driven by applied voltage

The effect of applying an external voltage to a molecular wire connecting t wo reservoirs of states is analyzed through the use of a Hubbard Hamiltonia n which explicitly depends on the applied voltage. The Hamiltonian is solve d using a method based on two-by-two rotations of molecular orbitals, which avoids divergence, and permits both ground and excited states to be obtain ed. The special case of a polyacene wire is studied in detail as a function of the wire length. It is shown that the energies of the ground and excite d states of the wire cross as the voltage is increased. This crossing produ ces specific features in the current-voltage characteristic of the molecula r wire.