Coarse-Grained Lattice Dynamics Calculations Combined with Independent Stiffness Approximation: A Comparative Study on Polymorphic Molecular Crystals

Abstract

We applied a previously reported lattice dynamics calculation scheme based on molecular vibration analysis, combining a coarse-graining (CG) scheme with an independent stiffness approximation (ISA) method, to polymorphs of p-dichlorobenzene crystals as a test case. To extend the applicability of the theoretical framework to complex crystal structures, we exploited the molecular symmetry within the lattice so that the ISA could effectively reduce the computational cost to a manageable level. We carefully examined the influence of the coupling between phonon modes on the accuracy of their frequencies under a truncated dimension of the CG space. Having obtained the phonon band structures of each of the three polymorphs, their thermodynamic functions were calculated, which demonstrated a subtle but meaningful difference originating from the crystal structure.