  There is renewed interest in the structure of the essential amino acid
Phenylalanine in the solid state. Three new polymorphs were found in the years
2012 to 2014. Here, we investigate the structure, stability, and energetical
ordering of these phases using first-principles simulations at the level of
Density Functional Theory incorporating van-der-Waals interactions. Two of the
distinct crystal forms are found to be structurally similar and energetically
very close after vibrational free energy corrections have been taken into
account. Infrared absorption spectra are likewise calculated and compared to
experimental measurements. By combining measurements obtained with a commercial
Fourier Transform Infra-Red spectrometer and a homemade air-photonics-based THz
Time Domain spectrometer, we could carry out this comparison in the vibrational
frequency region from 1 to 40~THz. The excellent agreement of the line
positions and the established energy ranking allow us to identify the most
stable polymorph of Phenylalanine.
