Thiophene chains in the vacuum or in weakly perturbing environments adopt the energetically most stable all-trans conformation. On metal surfaces, such as gold, the interaction with their image potential may modify the picture, as is now being shown by a number of experiments. We use a model that was parametrized to accurately reproduce the energy of adsorption/desorption of a number of organic molecules on gold to investigate the stability of the 20 rotamers of sexithiophene on this metal. We find that electrostatic interactions modify the stabilities of the individual molecules. Adsorption of dimers tends to stabilize the deposition of the all-trans co-conformer. We also find that the mobility of the rotamers on the surface is not directly connected to the interaction energy.