Characterizing Gateway Modes for Solid-Solid Phase Transitions in Organic Crystals: The Thermosalient 4-DBpFO.

We disclose the nature of the 4-DBpFO thermosalient (or "jumping") crystal phase transition 40 cm-1 (i.e., 1.2 THz) "gateway" mode using computed low-frequency Raman spectra (LFRS) benchmarked to experimental spectra and show that it is a coherent/in-phase collective motion of the inner-outer rings. We explore and reveal the roles of anharmonicity, thermal disorder, and volume fluctuations in determining the THz spectral shape. The anharmonic couplings and cell fluctuations appear to be more than just innocent spectators, and indeed, the intermolecular correlations for this mode involve multiple cells. To obtain this information, we use our Activity Weighted Velocities (AWV) method to integrate classical trajectories with the accuracy of hybrid Density Functional Theory (DFT) Raman activities. We demonstrate that this combination effectively computes accurate LFRS and offers a way to predict the Raman activity of thermally disordered systems and entropically stabilized phases, where current methods (harmonic spectra and DFT-Molecular Dynamics) fail.