The pressure- and temperature-induced polymorphic crystal phase transitions of p-terphenyl (PTP) have been modeled using a modified PCFF interaction force field. Modifications of the interaction potential were necessary to simultaneously model both the temperature-induced phase transition at ambient pressure and the pressure-induced phase transition at low temperature. Although the high-temperature and high-pressure phases are both characterized by flattening of the PTP molecule, the mechanisms of the temperature- and pressure-induced phase transitions are different. At high temperature thermal energy exceeds the torsional barrier, resulting in a bimodal phenyl ring twist angle distribution that averages to zero. In contrast, compression of PTP at high pressure results in a static planar structure. At high pressure the compression of the unit cell is also characterized by large compression of the a lattice parameter and weak compression of c, but some expansion of the b lattice parameter. The expansion of the b lattice parameter is likely associated with pressure-induced soft mode behavior of some lattice vibrations as well as soft mode behavior of pseudolocal phonons associated with impurities in PTP. The crystallographic angles alpha, beta, and gamma also indicate a triclinic crystal phase above the critical phase transition pressure of P-c similar to 0.5 GPa at low temperature, suggesting a distinct phase separate from the monoclinic high-pressure phase at high temperature.