Can we predict the structure and stability of molecular crystals under increased pressure? First-principles study of glycine phase transitions.

Abstract

The aim of this study was to determine whether the periodic density functional theory (DFT) calculations can be used for accurate prediction of the influence of the increased pressure on crystal structure and stability of molecular solids. To achieve this goal a series of geometry optimization and thermodynamic parameters calculations were performed for γ-glycine and δ-glycine structures at different pressure values using CASTEP program. In order to perform most accurate geometry optimization various exchange-correlation functionals defined within generalized gradient approximation (GGA): PBE, PW91, RPBE, WC, PBESOL as well as defined within local density approximation (LDA), i.e. CAPZ, were tested. Geometry optimization was carried out using different dispersion correction methods (i.e. Grimme, TS, OBS) or without them. The linear response density functional perturbation theory (DFPT) was used to obtain the phonon dispersion curves and phonon density of states from which thermodynamic parameters, such as: free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) were evaluated. The results of the geometry optimization depend strongly on the choice of the DFT functional. Calculated differences between the free energy of the studied polymorphic forms at the studied pressure values were consistent with experimental observations on their stability. The computations of thermodynamic properties not only confirmed the order of stability of two studied forms, but also enabled to predict the pressure at which this order is reversed. The results obtained in this study have proven that the plane-wave basis set first principles calculations under periodic conditions is suitable for accurate prediction of crystal structure and stability. © 2018 Wiley Periodicals, Inc.