There are many numerical techniques for solving Maxwell's equations and each has their advantages and disadvantages. So far, I have not found one that is good for everything, but I am still looking!!
With this in mind, I could help you better if you can answer the following questions:
What device are you modeling?
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Does your device contain only dielectrics? Does it contain metals? Does it contain only thin wires? Does it contain only spherical particles? Is it periodic or of finite size? Is it random in some sense?
What are you trying to calculate?
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Are you trying to calculate the field transmitted through or reflected from a device (i.e. scattering)? Are you trying to calculate the discrete modes of a waveguide or resonator? Do you need to calculate and visualize the fields?
What are the physical dimensions?
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Is your device many wavelengths large? It is small? Is it highly subwavelength?
-Tip