  Excited states of the of the double-stranded DNA model
(A)$_{12}\cdot$(T)_{12} were calculated in the framework of the exciton theory.
The off-diagonal elements of the exciton matrix were calculated using the
transition densities and ideal dipole approximation associated with the lowest
energy $\pi\pi^{*}$ excitations of the individual nucleobases obtained from
TDDFT calculations. The values of the coupling calculated with the transition
density cubes (TDC) and ideal-dipole approximation (IDA) methods were found
significantly different for the small inter-chromophore distances. It was shown
that the IDA overestimates the coupling significantly. The effects of the
structural fluctuations were incorporated by averaging the properties of the
excited states over a large number of conformations obtained from the MD
simulations.
