1. A system comprising: a finite element model (FEM) circuit, configured to determine a FEM that models an implanted electrode and a tissue medium in which the electrode is implanted; a Fourier FEM solver circuit, communicatively coupled to the FEM circuit, configured to calculate a potential distribution in the tissue medium using information from the FEM circuit and a capacitive component of at least one of the electrode and the tissue medium; and a volume of activation (VOA) circuit, communicatively coupled to the Fourier FEM solver circuit, configured to provide a predicted VOA using the potential distribution and a neuron model.