CALCULATION OF SURFACE QUADRUPOLE FIELD, POLARIZATION, AND PYROELECTRICITY FOR ANTHRACENE AND PYRENE CRYSTALS

Surface quadrupole tenser sums have been calculated for unrelaxed (001), (010), and (100) surfaces of anthracene and pyrene, with the molecules treated as points. The surface has most effect on sums relating molecules in different surface layers. With the molecular quadrupole moments, the quadrupole sums give the surface electric fields, which reach 5 GV m(-1) for anthracene and 15 GV m(-1) for pyrene. The fields approach the bulk value by about the fourth molecular layer below the surface. With the molecular polarizability and surface dipole tenser sums, the electric fields give the surface polarizations, which reach 80 mC m(-2) for anthracene and 340 mC m(-2) for pyrene. With the crystal thermal expansion for anthracene, the polarizations give the surface secondary pyroelectricity, which is found to be of the order of 10 mu C m(-2) K-1 for all the surfaces. Treating the molecules as point submolecules at the center of each aromatic ring tends to reduce the anisotropy of the results somewhat.