  Effects of nuclear dynamics on the energetics of polythiophene relevant for
the performance of organic solar cells are studied for the first time. Nuclear
motions change the expectation values of frontier orbital energies and the band
gap by about 0.1 eV vs. values at the equilibrium geometry, which is expected
to have a significant effect on light absorption, charge separation, and donor
regeneration. A new molecular dynamics (MD) algorithm, which accounts for the
quantum nature of vibrations, is introduced. It reproduces effects of
temperature and deuteration which are lost in the standard MD. Inclusion of
quantized vibrations leads to a broadening of the band gap and of energy levels
by about 20% at 300K, while having little effect on their expectation values
(which change by up to 0.03 eV). Increase in temperature from 300K to 400K and
deuteration cause an additional broadening of the spectrum by about 26% and
21%, respectively.
