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There is renewed interest in the structure of the essential amino acidPhenylalanine in the solid state. Three new polymorphs were found in the years2012 to 2014. Here, we investigate the structure, stability, and energeticalordering of these phases using first-principles simulations at the level ofDensity Functional Theory incorporating van-der-Waals interactions. Two of thedistinct crystal forms are found to be structurally similar and energeticallyvery close after vibrational free energy corrections have been taken intoaccount. Infrared absorption spectra are likewise calculated and compared toexperimental measurements. By combining measurements obtained with a commercialFourier Transform Infra-Red spectrometer and a homemade air-photonics-based THzTime Domain spectrometer, we could carry out this comparison in the vibrationalfrequency region from 1 to 40~THz. The excellent agreement of the linepositions and the established energy ranking allow us to identify the moststable polymorph of Phenylalanine.

THz to far-infrared spectra of the known crystal polymorphs of Phenylalanine