To clarify: There is an electrostatic solver in the Los Alamos Poisson-Superfish package. While it is easy enough to get E-field plots, I cannot see a way of getting the capacitance between the electrodes.
I’ve done that with a different electrostatic solver, for two conductors, one at potential zero and the other at potential U. The values of the potentials are set on the surfaces of the conductors at the stage when you specify the boundary conditions of the simulation. One can then use Electrostatic Energy=C*U^2/2=IntegralOverVolume(eps_0*E^2*dV/2) and obtain C from there. E is the electrostatic field obtained by the solver and if the solver cannot do the integral then you need to export E in a table file and perform the integration with MATLAB or a similar package.