Terahertz spectroscopy of amino acid crystals based on dispersion-correction functional theory

The terahertz spectra of solid-state amino acids with different polarities were measured by terahertz time-domain spectroscopy and Fourier transform infrared spectroscopy. The measured absorption spectra were in good agreement with the results simulated by density functional theory using hybrid functional Becke-3-Lee-Yang-Parr and generalized-gradient approximation Perdew-Burke-Ernzerhof functional with the same basis set of 6-311 G (d, p), respectively. In order to construct a precise crystal structure, dispersion-corrected density functionals were included in the simulation. It was found that Becke-3-Lee-Yang-Parr functional combined with dispersion-correction could produce accurate simulation results that corresponded to the experimental terahertz spectra of solid-state amino acids. However, it greatly increased the time for calculation. The method of Perdew-Burke-Ernzerhof functional with dispersion-correction provided comparable accuracy to Becke-3-Lee-Yang-Parr with dispersion-correction, and much higher simulation speed. The spectral features are assigned as primarily external lattice translations and rotations with lesser contributions to intramolecular torsions.