The application of external forces to computational models of crystals

A computational model of a crystal consists of a description of the crystal structure, expressions for calculating the energy of the model in terms of this structure, and a way of minimizing this calculated energy with respect to the structural variables. External forces such as hydrostatic pressure, normal and shearing stresses, or electric fields can be simulated by adding a term to the energy which describes the external energy of the force-producing device. Minimizing the total energy produces a structure distorted by the external force. From the calculated distortion it is possible to obtain the complete elastic tensor and the dielectric and piezoelectric constants of the crystal. Some phase changes can be simulated, and it should be possible to induce ferroelastic or ferroelectric transitions in the model. The effect of these forces on the orientation and conformation of molecules in the crystal can also be computed.