1. A method of determining one or more profile parameters of a structure using an optical metrology model, the optical metrology model comprising a profile model, an approximation diffraction model, and a fine diffraction model, the method comprising: (a) using a processor, developing a metrology model of a structure, the metrology model including a profile model, the profile model having profile parameters; (b) optimizing the metrology model, the optimized metrology model including an optimized profile model, (c) calculating a simulated approximation diffraction signal based on the approximation diffraction model of the structure; (d) generating a set of simulated fine diffraction signals from a set of profile parameters, the simulated fine diffraction signals generated using the optimized profile model of the structure; (e) generating a library of difference diffraction signals and corresponding profile parameters, the difference diffraction signal calculated by subtracting the simulated approximation diffraction signal from each simulated fine diffraction signal of the set of simulated fine diffraction signal; (f) determining a best match of a measured diffraction signal adjusted by subtracting the simulated approximation diffraction signal, against the library of difference diffraction signals; and (g) if one or more matching criteria are met, determining at least one profile parameter of the structure; wherein the generated library of difference diffraction signals and corresponding profile parameters is saved in storage accessible to the processor and wherein the structure is a wafer structure.