Molecular simulations of clay minerals: a study considering the change of cell size and shape

A molecular simulation study of dehydrated 2:1 clay minerals is carried out using a new MD simulation method that is capable of simulating a system under the most general applied stress conditions by considering the changes of MD cell size and shape. The tensor defining the cell size and shape is correlated with the atomic level stress tensors (both internal and external) through a Lagrangian formulation. In this paper, the static version of the method has been applied for the first time to the simulations of dehydrated mica sheets and has successfully revealed unforeseen structural transformations of clay minerals upon relaxation under different external stress conditions. Simulation results show that the degrees of freedom that the simulation cell possesses (i.e., whether the cell size or shape change is allowed) determines the final equilibrated crystal structure of clay minerals. When full allowance is given to the cell size and shape change, large shear distortions of clay minerals occur, resulting in the eliminations of interlayer spacing and internal shear stresses. However, when only the cell size change is allowed, interlayer spacing is retained, but large internal shear stresses also exist.