Non-convex optimization based on structural phase transformations in crystal lattice of materials undergoing symmetry changes. These transformations induce an energy density function dependent on elastic deformations; here the energy density is non-convex. Minimization of a bulk energy function of J. L. Ericksen and R. D. James, discussion of drawbacks from use of square mesh discretization, and of advantages of triangular mesh. Numerical examples. Appendix on tensor matrix calculations.