Molecular Modeling of Crystalline Oligothiophenes: Testing and Development of Improved Force Fields

We present the results of an extensive test of different force field models for crystalline oligothiophenes. The
models are mostly based on MM3 for intramolecular degrees of freedom, but they rely on ab initio calculations
for the inter-ring torsion potential and the molecular charge distribution. The latter is represented using either
point charges from a fit of the molecular electrostatic potential or atomic charges, dipoles, and quadrupoles
from a distributed multipole analysis. The force fields are tested by comparing the experimental structures
with static energy minimizations and, in a few cases, room-temperature molecular dynamics simulations,
also. We find that the point-charge model yields satisfactory results for most systems, including
α-tetrathiophene, α-sexithiophene, tetrahexylsexithiophene, and bis(dithienothiophene). However, α-perfluorosexithiophene represents one difficult case where the distributed multipole model turns out to be clearly
superior. Finally, we find that it is necessary to rescale the MM3 van der Waals parameters when these are
employed in molecular dynamics simulations to reproduce the correct crystal densities.