PETSc Itererative solver combinations (i.e. preconditioner and iterative methods). Stick with default solver (i.e. MUMPS) for stability. Try one of the highlighted solvers (from the paper) for a larger model or faster solution. CAUTION: iterative methods are sensitive to boundary conditions (i.e. multipoint constraints can cause convergence problems). Feel free to customize these templates with specific PETSc options.