Rotational potentials are computed for heptamers and nonamers of an amino, nitro-substituted phenylene ethynylene trimer molecule. A herringbone and a parallel-slipped packing arrangement are considered. The effect of tilting the molecules with respect to the surface as well as the effect of the gold support are also taken into account. The herringbone structure with the molecules perpendicular to the surface has a low rotational barrier (2 kcal/mol). Tilting the molecules by 30° increases the rotational barriers significantly (16 kcal/mol). The parallel-slipped structure has rotational barriers of 7 kcal/mol. Including the effect of the gold support increases the rotational barriers for tilted molecules but has little effect on perpendicular molecules.