1. A method operating on a computer of synthesizing a photolithographic data set comprising: partitioning by using the computer the photolithographic data set into a first part and second part based on criticality, wherein the photolithographic data set provides a target pattern that produces a wafer pattern, the wafer pattern is calculated at a given dose and defocus condition, and the first part is more critical than the second part; using a first computational model to calculate a first figure-of-merit of the first part, wherein the first figure-of-merit is a measure of a first difference between sections of the target pattern and the wafer pattern that correspond to the first part; changing the first part of the photolithographic data set to increase said first figure-of-merit; using a second computational model to calculate a second figure-of-merit of the second part, wherein the second computational model executes faster than said first computational model, and the second figure-of-merit is a measure of a second difference between sections of the target pattern and the wafer pattern that correspond to the second part; and changing the second part of the photolithographic data set to increase the second figure-of-merit; wherein the photolithographic data set corresponds to position-dependent variations in an optical refractive index n(x, y, z) of at least a part of a photomask; and wherein said operation of using a first computational model to calculate a first figure-of-merit of the photolithographic data set includes: using said first computational model to calculate a scattering of illuminating electromagnetic waves using a coupled wave analysis, a boundary integral equation, a finite-elements method, a finite-difference method, or a thin-film interference method.