Two different approaches were used to calculate the infrared absorption spectra of the various forms of cysteine. From the static calculations using VASP the Born charges and phonon frequencies at the Γ point were determined, enabling the calculation of the infrared and terahertz spectrum by the PDielec package, which assumes a powdered crystalline material of spherical morphology (10% by volume) supported in a PTFE matrix, which was represented using the Maxwell-Garnett effective medium approximation. Unless otherwise stated a line broadening of 5 cm -1 was used in all calculations of the terahertz and infrared spectra from the static DFT calculations. For unit-cells with 4 or less molecules in the cell, the calculation of the phonon frequencies was performed using Density Functional Perturbation Theory implemented in VASP. For larger cells the Phonopy 34 package was used with VASP providing the energies and forces to calculate the dynamical matrix numerically with a summary of parameters used included in the SI (Section 3). VASP was used to calculate the Born charges of the cells to determine the infrared intensity of the modes, for all cells, independent of size.