l-Serine Anhydrous Crystals: Structural,Electronic, and Optical Properties by First-Principles Calculations,and Optical Absorption Measurement.

The X-ray diffraction data of l-serine anhydrous crystalswas taken into account to initialize the total energy minimizationprocess of their unit cell through density functional theory (DFT)computations, which were performed within both the local density andgeneralized gradient approximations with dispersion, LDA, and GGA+D,respectively. The calculated lattice parameters are in good agreementwith the experimental results for the dispersion corrected generalizedgradient approximation functional, with a unit cell volume largerby only about 0.32%; the Mulliken and Hirschfield charges show thezwitterionic state of the l-serine molecules in the DFT convergedcrystals. The electronic (band structure, density of states) and opticalabsorption properties were calculated to explain the light absorptionof the l-serine anhydrous crystalline powder we have measuredat room temperature. The optical absorption related to transitionsbetween the top of the valence band and the bottom of the conductionband involves O-2p valence states and H-1s conduction states. TheLDA (4.74 eV) and GGA+D (4.75 eV) estimated energy gaps are about1 eV below the estimated value from optical absorption measurements(5.90 eV). Small values were obtained for the electron effective masses,which are almost isotropic, whereas large anisotropic values werefound for hole effective masses, suggesting that the l-serineanhydrous crystal behaves like an n-type wide gap semiconductor. Differentdielectric function profiles obtained for some of the most importantsymmetry directions also demonstrate the optical anisotropy of l-serine anhydrous crystals. [ABSTRACT FROM AUTHOR]