1. A computer-implemented method for creating a model of a lithography process, comprising: defining a set of model parameters and a set of fitting parameters having variable values for the model, the set of model parameters defining a process window for the lithography process in a parameter space; selecting a set of initial values for the set of fitting parameters; selecting a plurality of sampling locations within the process window, the plurality of sampling locations being a subset of all possible process conditions within the process window; generating, using the computer, simulated results of the lithography process at each of the plurality of sampling locations within the process window using the model with the set of initial values by simulating the lithography process with varying values of the model parameters corresponding to the plurality of sampling locations within the process window while holding constant the initial values; comparing, using the computer, the simulated results with actual results of the lithography process at the plurality of sampling locations within the process window to produce a total difference measure between the simulated results and the actual results at the plurality of sampling locations; modifying, using the computer, the set of fitting parameter values and generating additional simulated results at each of the plurality of sampling locations within the process window to identify optimum fitting parameter values such that the total difference measure between the actual results and simulated results produced using the optimum fitting parameter values is below a predetermined threshold; and finalizing the model by including the optimum fitting parameter values in the model, the finalized model being capable of simulating the lithography process at any location within the entire process window.