1. A computer-implemented method for determining the volume of activation of neural tissue, comprising: having one or more parametric equations that define a volume of activation, wherein the parameters for the one or more parametric equations are given as a function of an input vector that includes stimulation parameters; receiving input data that includes values for the stimulation parameters and defining the input vector using the input data; applying the input vector to the function and obtaining the parameters for the one or more parametric equations; solving the parametric equation to calculate the volume of activation; and displaying the calculated volume of activation on a display screen.