The influence of Coulombic interactions on thermal parameters for naphthalene and anthracene: a lattice-dynamical approach

A lattice-dynamical calculation of thermal parameters is performed for naphthalene and anthracene in the external Born-von Karman formalism using 6-exp potential functions and electrostatic interactions in the form of an atomic point-charge model. Phonon dispersion curves and density-of-states functions are calculated with and without charges. In general, the effect of the Coulombic forces is small and sensibly affects optical branches linked to the B_u mode in both compounds, improving the agreement with the experimental data. The effect on thermal parameters is roughly opposite to that of molecular non-rigidity.