  We present a joint theoretical and experimental study to investigate
polymorphism in $\alpha$-sexithiophene (6T) crystals. By means of
density-functional theory calculations, we clarify that the low-temperature
phase is favorable over the high-temperature one, with higher relative
stability by about 50 meV/molecule. This result is in agreement with our
thermal desorption measurements. We also propose a transition path between the
high- and low-temperature 6T polymorphs, estimating an upper bound for the
energy barrier of about 1 eV/molecule. The analysis of the electronic
properties of the investigated 6T crystal structures complements our study.
