van der Waals Phonons in Molecular Crystals

Low-frequency terahertz (THz) vibrations are ubiquitous in molecular materials throughout chemistry and biology [e.g. PRL 113, 055701; PRL 119, 097404]. Such delocalized vibrations play a key role in many phenomena, including entropic stabilization of molecular crystal polymorphs, protein folding, and signaling. However, our understanding of mechanisms behind THz vibrations in molecular materials is far from complete. Here we study a range of molecular crystals with DFT including many-body treatment of van der Waals (vdW) dispersion interactions (DFT+MBD). We find that THz vibrations are very sensitive to the correct treatment of electrostatic effects in DFT and explicit quantum-mechanical treatment of vdW interactions. We discuss a way to categorize vibrational modes in the THz range and show that these vibrations are crucial to achieve a better understanding of molecular materials.