1. A computer system comprising a processor that comprises: a finite element model (FEM) circuit, configured to determine a FEM that models an implanted electrode, a tissue medium in which the electrode is implanted, and an electrode-tissue interface; a Fourier FEM solver circuit communicatively coupled to the FEM circuit and 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, the tissue medium, and the electrode-tissue interface, wherein the calculating includes: separating a stimulus waveform into frequency components; obtaining a solution of a Poisson equation at at least one frequency component with a stiffness matrix (σ+iω), wherein the stiffness matrix is representative of the capacitive component of the at least one of the electrode, the tissue medium, and the electrode-tissue interface; and using the solution to reassemble the stimulus waveform; 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.