  Charge transfer can take place along double helical DNA over distances as
long as 30 nanometers. However, given the active role of the thermal
environment surrounding charge carriers in DNA, physical mechanisms driving the
transfer process are highly debated. Moreover, the overall potential of DNA to
act as a conducting material in nanoelectronic circuits is questionable. Here,
we identify key principles in DNA nanoelectronics by performing an exhaustive
computational study. The electronic structure of double-stranded DNA is
described with a coarse-grained model. The dynamics of the molecular system and
its environment is taken into account using a quantum scattering method,
mimicking incoherent, elastic and inelastic effects. By analyzing all possible
sequences with 3 to 7 base pairs, we identify fundamental principles in DNA
nanoelectronics: The environment crucially influences the electrical
conductance of DNA, and the majority of sequences conduct via a mixed,
coherent-incoherent mechanism. Likewise, the metal-molecule coupling and the
gateway states play significant roles in the transport behavior. While most
sequences analyzed here are exposed to be rather poor electrical conductors, we
identify exceptional DNA molecules, which we predict to be excellent and robust
conductors of electric current over a wide range of physical conditions.
