1. A computer-executed method for determining a process model for an extreme ultra violet (EUV) lithography process, the method comprising: receiving measured critical dimension (CD) data which was obtained by measuring CDs of features that were printed by applying the EUV lithography process to a mask layout; receiving an uncalibrated process model for the EUV lithography process, wherein the uncalibrated process model includes a mask error modeling term which is based at least on an aerial image sensitivity to mask modifications which represent mask errors; and determining the process model for the EUV lithography process by fitting the uncalibrated process model using the measured CD data and the mask layout so that CDs predicted by the process model substantially match the measured CDs.