Crystal and molecular structures of the polymorphic para-dichlorobenzene p-C6H4Cl2 (p-DCB) were investigated by molecular packing analysis with a computed dynamical model. This model includes anisotropic atomic repulsive interactions, molecular deformations, and thermal motions. Several crystalline structures with triclinic P (1) over bar, monoclinic P2(1)/a (and P2(1)/c) symmetry have been found by the calculation. Calculated crystal and molecular parameters agree with data of the three phases beta, alpha, and gamma determined by x-ray diffraction between 300-100 K at ambient pressure. At room temperature various crystal structures with orthorhombic Pbca and Abma symmetry calculated in the pressure range of a fourth pressure-induced phase delta are compared with the experimental structure delta(0) proposed for this phase. Among them the orthorhombic Abma structure named delta(s) has the lowest energy and enthalpy. It is suggested as a possible structure for the high pressure phase delta. (C) 2003 American Institute of Physics.