Low Frequency Neutron Spectroscopy of Organic Molecular Solids and Parallel First-Principles Calculations*

Vibrational contributions are essential to thermodynamic properties of materials. We present spectra of organic molecular crystals and polymorphs using the next-generation, high-signal VISION spectrometer in the far-infrared (FIR) and mid-infrared (MIR) range. Different spectral signatures were found for polymorphic forms, and numerous modes undetectable with optical methods are observed. In addition to correct calculation of peak positions, highly accurate line-shape features are reproducible by simulation using solid-state density functional theory (DFT) vibrational calculations on OLCF resources. High confidence spectral assignments and thermodynamic information are thus obtainable, and new benchmarks for DFT calculations are provided. Detection of soft modes provides information for theoretical exploration of phase transitions based on experiment.