  Molecular clusters of 1,3-butadiene were theoretically investigated using a
variety of approaches, encompassing classical force fields and different
quantum chemical (QC) methods, as well as density-functional based
tight-binding (DFTB) in its self-consistent-charge (SCC) version. Upon suitable
reparametrization, SCC-DFTB reproduces the energy difference and torsional
barrier of the trans and gauche conformers of the 1,3-butadiene monomer
predicted at the QC level. Clusters of pure trans and gauche conformers
containing up to 20 monomers were studied separately, their energy landscapes
being explored using the force fields, then locally reoptimized using DFT or
SCC-DFTB. The all-trans clusters are generally found to be lower in energy and
produce well-ordered structures in which the planar molecules are arranged
according to a herringbone motif. Clusters of molecules in the gauche
configuration are comparatively much more isotropic. Mixed clusters containing
a single gauche molecule were also studied and found to keep the herringbone
motif, the gauche impurity usually residing outside. In those clusters, the
strain exerted by the cluster on the gauche molecule leads to significant
geometrical distortion of the dihedral angle already at zero temperature.
Finally, the finite temperature properties were addressed at the force field
level, and the results indicate that the more ordered all-trans clusters are
also prone to sharper melting mechanisms.
