A method for performing model based optical proximity correction (MBOPC) and a system for performing MBOPC is described, wherein the process model is decomposed into a constant process model term and a pattern dependent portion. The desired wafer target is modified by the constant process model term to form a simulation target that is used as the new target within the MBOPC process. The pattern dependent portion of the model is used as the process model in the MBOPC algorithm. This results final mask designs that result in improved across-chip line width variations, and a more robust MBOPC process.