  We introduce a powerful Monte Carlo (MC) algorithm for the atomistic
simulation of bulk models of oligo- and poly-thiophenes by redesigning MC moves
originally developed for considerably simpler polymer structures and
architectures, such as linear and branched polyethylene, to account for the
ring structure of the thiophene monomer. Elementary MC moves implemented
include bias reptation of an end thiophene ring, flip of an internal thiophene
ring, rotation of an end thiophene ring, concerted rotation of three thiophene
rings, rigid translation of an entire molecule, rotation of an entire molecule
and volume fluctuation. In the implementation of all moves we assume that
thiophene ring atoms remain rigid and strictly co-planar; on the other hand,
inter-ring torsion and bond bending angles remain fully flexible subject to
suitable potential energy functions. Test simulations with the new algorithm of
an important thiophene oligomer, {\alpha}-sexithiophene ({\alpha}-6T), at a
high enough temperature (above its isotropic-to-nematic phase transition) using
a new united atom model specifically developed for the purpose of this work
provide predictions for the volumetric, conformational and structural
properties that are remarkably close to those obtained from detailed atomistic
Molecular Dynamics (MD) simulations using an all-atom model. The new algorithm
is particularly promising for exploring the rich (and largely unexplored) phase
behavior and nanoscale ordering of very long (also more complex)
thiophene-based polymers which cannot be addressed by conventional MD methods
due to the extremely long relaxation times characterizing chain dynamics in
these systems.
