Terahertz spectroscopy of vibrational anharmonicity in crystalline γ-aminobutyric acid

Vibrational anharmonicity in molecular solids is of key importance for fundamental physical properties such as thermal expansion and specific heat capacity. In solids, anharmonic character is largely dictated by intermolecular forces, and can be assessed using terahertz vibrational spectroscopy to probe the sub-100cm^-1 motions. Significant anharmonicity can be readily identified by anomalous red shifting of terahertz spectral peaks with sample cooling, as seen in the neurotransmitter γ-aminobutyric acid (GABA). The potential energy surface of the lowest-frequency lattice vibration in crystalline GABA was investigated in this work using solid-state density functional theory, which revealed a large deviation from harmonic character. A method for accounting for such anharmonic motion in crystals is presented based on numerical sampling of the potential energy surfaces and the quasi-harmonic approximation (QHA).