  When the DNA double helix is subjected to external forces it can stretch
elastically to elongations reaching 100% of its natural length. These
distortions, imposed at the mesoscopic or macroscopic scales, have a dramatic
effect on electronic properties at the atomic scale and on electrical transport
along DNA. Accordingly, a multiscale approach is necessary to capture the
electronic behavior of the stretched DNA helix. To construct such a model, we
begin with accurate density-functional-theory calculations for electronic
states in DNA bases and base pairs in various relative configurations
encountered in the equilibrium and stretched forms. These results are
complemented by semi-empirical quantum mechanical calculations for the states
of a small size [18 base pair poly(CG)-poly(CG)] dry, neutral DNA sequence,
using previously published models for stretched DNA. The calculated electronic
states are then used to parametrize an effective tight-binding model that can
describe electron hopping in the presence of environmental effects, such as the
presence of stray water molecules on the backbone or structural features of the
substrate. These effects introduce disorder in the model hamiltonian which
leads to electron localization. The localization length is smaller by several
orders of magnitude in stretched DNA relative to that in the unstretched
structure.
