How to program the FDTD for EM Simulations

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Let's see how we are going to make a grid.In here I will not tell you how to find a geometry modeler or how to find or make a mesher.In here I consider that somehow you have solved that problem. In this guide we consider the mesh to
have the following form:
The mesh is a 3D array where each value in the array represents a cell of the geometry and the surrounding vacuum if there is any.
What we need for an FDTD simulation is to know the dielectric parameters for the material of every single cell.
For this simulation we will not consider magnetic losses so we will need , e m s ,for every cell.