==Defining The Physical Structure==
[[Image:FDTD1.png|thumb|left|200px|[[FDTD Module]]'s Navigation Tree.]]
In [[EM.Cube]]'s [[FDTD Module]], a physical structure consists of sets of objects that are grouped together and identified by their material types. Materials are divided into seven categories that are listed under the '''Physical Structure''' node at the top of the Navigation Tree: