1. A method comprising: calculating a transmission cross coefficient (TCC(n)) in a spatial domain for a simulation of a mask pattern to provide a first result; calculating a transmission cross coefficient TCC(x 1 , y 1 , x 2 , y 2 ) in the spatial domain utilizing the first result to provide a second result; and calculating intensity at x and y coordinates of a wafer (i.e., I(x, y)) in a spatial domain utilizing the second result to provide a third result.