1. A method of calibrating a photolithographic system, the method being implemented by a computer, comprising: obtaining a plurality of measured dimensions of circuit patterns generated by the photolithographic system using a configuration of a photolithographic process, wherein the configuration of the photolithographic process includes a plurality of values of parameters related to the projection of an image associated with the circuit patterns by the photolithographic system; generating, using the computer, a plurality of estimated dimensions of the circuit patterns using a model of the configuration of the photolithographic process; applying a polynomial expansion that approximates changes in the estimated dimensions based on changes in values of the parameters of the configuration; and calibrating, using the computer, the photolithographic process based on the polynomial expansion, wherein calibrating the photolithographic process includes the step of minimizing differences between the estimated and measured dimensions using an optimization algorithm, and wherein the calibrated photolithographic process includes a plurality of calculated values of the parameters.