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We investigate the charge transfer characteristics of one and two excesscharges in a DNA base-pair dimer using a model Hamiltonian approach. Theelectron part comprises diagonal and off-diagonal Coulomb matrix elements sucha correlated hopping and the bond-bond interaction, which were recentlycalculated by Starikov [E. B. Starikov, Phil. Mag. Lett. {\bf 83}, 699 (2003)]for different DNA dimers. The electronic degrees of freedom are coupled to anohmic or a super-ohmic bath serving as dissipative environment. We employ thenumerical renormalization group method in the nuclear tunneling regime andcompare the results to Marcus theory for the thermal activation regime. Forrealistic parameters, the rate that at least one charge is transferred from thedonor to the acceptor in the subspace of two excess electrons significantlyexceeds the rate in the single charge sector. Moreover, the dynamics isstrongly influenced by the Coulomb matrix elements. We find sequential and pairtransfer as well as a regime where both charges remain self-trapped. Thetransfer rate reaches its maximum when the difference of the on-site andinter-site Coulomb matrix element is equal to the reorganization energy whichis the case in a GC-GC dimer. Charge transfer is completely suppressed for twoexcess electrons in AT-AT in an ohmic bath and replaced by damped coherentelectron-pair oscillations in a super-ohmic bath. A finite bond-bondinteraction $W$ alters the transfer rate: it increases as function of $W$ whenthe effective Coulomb repulsion exceeds the reorganization energy (invertedregime) and decreases for smaller Coulomb repulsion.

Multiple-charge transfer and trapping in DNA dimers