 The FDFD method requires solving a sparse matrix, which even for simple problems can be 20,000 by 20,000 elements or larger, with over a million unknowns.In this respect, the FDFD method is similar to the finite element method, which is a finite integral method and is also usually implemented in the frequency domain.There are efficient numerical solvers available so that matrix inversion—an extremely computationally expensive process—can be avoided.