Another effort to combine the molecular symmetry into the phase transition temperature is to pursue theoretical consistency. , - Within a homologous series, adding a substituent that increases internal cohesive forces also increases densities, boiling points, heat of vaporization, refractive indexes, and surface tensions. However, the melting points are the outstanding exceptions, which behave idiosyncratically. The lack of molecular symmetry in the functional group contribution equations is considered a reason.