Elasticity, Bulk Modulus, and Mode Grüneisen Parameters of the HPTB Molecular Crystal:  Computational Investigation of a Clathrate Precursor

Harmonic lattice dynamical calculations in the rigid-body approximation are employed to obtain frequencies of normal modes in the region of the Brillouin zone center for the hexakis(phenylthio)benzene (HPTB) molecular crystal. The intermolecular interaction for an appropriately optimized lattice structure is defined by the Buckingham potential with pertinent parameters. The velocities of the acoustic phonons are used to derive the full set of elastic constants and then the bulk modulus. Homogenous deformation of the crystal lattice provides frequency changes of the Raman active modes which allows calculation of selected anisotropic mode Gruneisen parameters. Relaxation of the internal strain is used for a realistic modeling of tire crystal response to the external strains and stresses. The anharmonicity and anisotropy of the intermolecular interaction are assessed from the derived parameters.