## Changing Solution Parameters

After we have defined the material properties, we need to select the proper solver for our problem. By default, pdnmesh will solve a Poisson equation. However, we need to use the edge element solver now. Hence, from the menubar we select
->. Next, we change the equation type from *Poisson* to *Inhomogeneous Helmholtz*. You should also give the number of eigenmodes you want to solve. Finally, select -> to solve the problem.

I should remind you that if you do not have LAPACK, the solver will not work. Also remember that unlike solving a Poisson equation, solving a generalised eigenvalue problem (especially a non positive definite one) will take some time and will also use more memory. So be patient until it finishes. If your computer cannot handle it, reduce the number of triangles of the mesh.

From versions 0.2.2 onwards, pdnMesh supports the ARPACK solver. If your problem size is large, it is highly recommended to use this solver for eigenvalue problems. This is an iterative solver that uses very little memory. However, it would take longer to find the solution, compared with the full matrix solvers. So please be patient.