Comparative Study of Force Fields for Molecular Dynamics Simulations of α-Glycine Crystal Growth from Solution

The study of crystal growth using molecular dynamics is very difficult due to the slow rate of growth and the complex intermolecular interactions involved. In order to perform molecular dynamics simulations of crystal growth from aqueous solution accurately, both interactions within the crystal and interactions between the crystal molecules and water need to be described correctly. In this study, we have investigated and compared various force fields for their applicability in the molecular dynamics study of α-glycine crystallization. The force fields that have been investigated include Charmm27, general AMBER, OPLS-AA/L, and Gromos53a6. As the electrostatic interactions are expected to play a significant role in the properties of bulk glycine crystal and solution, five other charge sets obtained from first principles calculations have also been investigated. Simulations in the bulk crystal and aqueous solution environments have been carried out, and results for the α-glycine lattice energy, solution densities, and self-diffusivities are compared to available experimental results. The solution enthalpy has also been determined and is found to be a good indicator of the applicability of the force field for crystal growth studies. The general AMBER force field, coupled with charges derived from calculations using the Complete Neglect of Differential Overlap method, is found to be the optimal force field, resulting in significant crystal growth at the (010) face of the α-glycine crystal from a slightly supersaturated glycine solution. Such simulations provide insights into the mechanism of crystal growth at the molecular level. We observe that α-glycine crystal growth involves monomeric growth units that attach to the existing crystal face in the correct molecular orientation. This work also opens up possibilities to systematically investigate the various factors that affect crystal growth through simulations, such as temperature, concentration, solvent, impurities, etc., so as to gain a better overall understanding of the crystal growth process.