Doesn't the Eigenmode solver require resonant frequencies? As it is a parallel plate capacitor, the resonances do not seem like they would be realistic (i.e. the inductance would be incredibly small).
Concerning the calculation, I think this is the crux of my question, how do I go about that calculation? While I may know the Efield, and thus should be able to calculate C = Q/integ(Efield(s), s). I don't understand how to ascribe the line of integration (for elements dS) or how to determine Q (maybe, integrating the Jfield across the parallel plate surface?).
Any help along these lines (pun intended) would be greatly appreciated.