1. A method of characterizing a plurality of structures of interest on a semiconductor wafer, the method comprising: generating a plurality of models that are different from each other in that they have different combinations of floating and fixed critical parameters for outputting simulated spectra, wherein the plurality of models are generated as final models to determine different one or more critical parameters for unknown structures based on spectra collected from such unknown structures; after generating the models and without generating another model, determining which one of the models best correlates with each critical parameter based on reference data that includes a plurality of known values for each of a plurality of critical parameters and corresponding known spectra, for spectra measured from an unknown structure using a metrology tool, selecting and using different ones of the models to determine different ones of the critical parameters of the unknown structure based on determining which one of the models best correlates with each critical parameter based on the reference data; and reporting the different ones of the determined critical parameters on a display of the metrology tool.