The computation of the capacitance coefficients for alternative interconnect configurations requiring repeated calculations as the configurations are changed can be substantially accelerated by using an "exterior template", that is, a set of "template capacitors" connected to the surface of the smallest rectangular region encompassing the planned interconnects. These template capacitors represent the effect of the medium for the fringing field outside the rectangle, and they eliminate the need to sample the fringing field every time a new calculation is made. The template capacitors can be determined by using the theory of infinite grids to eliminate almost entirely the medium-truncation error. All this works for both two-dimensional and three-dimensional (3-D) configurations, and it is especially advantageous in the 3-D case.