1. A tangible machine-readable medium comprising instructions, which when executed by one or more processors perform the following operations: calculating a transmission cross coefficient (TCC(n)) in a frequency domain for a simulation of a mask pattern to provide a first result; calculating an intensity component I n (x, y) in a frequency domain utilizing the first result to provide a second result; calculating an intensity at a location (x, y) utilizing the second result to provide a third result; evaluating the third result to determine if the intensity will result in the mask pattern being written onto a wafer; and calculating the intensity at location (x, y) using a plurality of defocus values, wherein the intensity at location (x, y) is calculated with an equation I ⁡ ( x , y ) = ∑ n = 0 N cut ⁢ ( - ⅈπ ) n n ! ⁢ z n ⁢ I n ⁡ ( x , y ) , where n is an integer and N cut is a number greater than three (3).