Size-Reduction of Phonon Band Calculation for Coarse-Grained Molecular Crystals Using “Independent Stiffness Approximation”

We propose a new computational scheme for calculating the phonon band diagrams of molecular crystals. In conventional computational models, the vibrational unit is typically an atom, which leads to high computational costs and less intuitive vibrational mode analyses, especially when dealing with molecular crystals. In the current study, we utilized a coarse-graining (CG) scheme that treats molecules as vibrational units, reducing the number of intramolecular vibration branches. To apply this approach to practical lattice calculations, we developed a method called independent stiffness approximation (ISA), which enables the construction of an approximate dynamical matrix by assembling small partial matrices. By integrating ISA with the CG approach, we conducted calculations and analyses of the β-phase of dichlorobenzene as a case study and validated the reliability of the proposed method.