Two existing force fields were adjusted for optimal reproduction of the crystal structures and lattice energies of halogenated benzenes and naphthalenes. Crystal structure predictions, including structures with two independent molecules, were made for 21 compounds. It was found that the two force fields, with different functional forms, led to comparable values for the energy of the experimental structures with respect to the global energy minima. These force fields were used for the prediction of disorder in para-substituted benzenes, without using other experimental data. This was done by artificially creating disorder in the most promising hypothetical ordered structures, and comparing the excess energy with RT ln 2. Predictions of the occurrence of orientational disorder in asymmetrically substituted compounds were in agreement with experimental evidence. A comparable approach was followed to study the possibility of mixed crystals consisting of two symmetrically substituted para-dihalobenzenes. It appeared to be possible to estimate the randomization energy for mixtures consisting of two isomorphic compounds. However, the question whether or not such isomorphic compounds will exist in the real world could not be answered with the currently available force fields and methods.