  We studied the Soret effect in binary dimer-monomer mixtures using
non-equilibrium molecular dynamics simulations and investigated the pure
contribution of the internal degree of freedom of flexible molecules to the
Soret effect. We observed that the thermal diffusion factor tends to decrease
and change its sign as the molecules become softer. We proposed two possible
mechanisms of our observations: change of the molecule structures with the
temperature, causing bulkier molecules to migrate to the hotter region;
asymmetry of the restitution between rigid and flexible molecules, due to which
flexible molecules show larger restitution when placed at the hotter region.
