Combined Next-Generation Neutron Vibrational Spectroscopy and High-Accuracy Massively Parallel DFT Calculations Benchmark Electronic Descriptions of Complex Organic Molecular Systems

Vibrations in crystals govern their fundamental properties. In organic molecular crystals, low frequency vibrations depend on intermolecular interactions which are difficult to describe accurately with density functional theory (DFT) and must be carefully benchmarked. Inelastic neutron scattering (INS), complemented by accurate theoretical studies, can provide vital information on vibrations and molecular forces. Next-generation INS instruments measure THz and far-IR regions with excellent resolution; the VISION spectrometer is unique in providing high signal allowing for high-quality low-frequency data. Impressive agreement with experimental spectra is obtained using DFT calculations, however, discrepancies still exist. Frequency errors lead to incorrect Helmholtz free-energy estimates used in predictions of the relative stability of crystals, and incorrect intensities indicate poorly described intermolecular forces. By exploiting the parallel nature of finite-displacement methods, we can calculate the spectra of complex pharmaceutical and bio-organic solids using very large basis sets and a number of DFT functionals and dispersion descriptions in a few hours using the Titan supercomputer. Such benchmarks can transform theoretical studies of organic materials.