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

Abstract

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 Grüneisen parameters. Relaxation of the internal strains is used for a realistic modeling of the crystal response to the external strains and stresses. The anharmonicity and anisotropy of the intermolecular interaction are assessed from the derived parameters.

Elastic Constant Calculations for Molecular Organic Crystals

“…Several methodologies have been used to calculate elastic constants from the second derivatives of the potential energy surface at the classical, 0 K structure. These methods include full lattice dynamic calculations, which involve extracting wave velocities from calculated phonon dispersion curves and relating these to the elastic constants, − the numerical application of axial and shear strains, and analytic expressions directly relating second derivatives of the potential energy hypersurface to the elastic constant matrix. − We have incorporated the analytic method for rigid molecular crystals modeled by atom−atom interactions into the crystal structure modeling program DMAREL, thus enabling the use of highly accurate, anisotropic electrostatic models in elastic constant calculations.…”