1. A computer-executed method for determining an improved process model which models mask corner rounding effects, the method comprising: receiving a mask layout; receiving process data which was generated by applying a photolithography process to the mask layout; receiving an uncalibrated process model; identifying a set of corners in the mask layout; modifying the mask layout in proximity to the set of corners to obtain a modified mask layout, wherein the modifications to the mask layout relate to mask corner rounding effects; and calibrating, by one or more computers, the uncalibrated process model using the modified mask layout and the process data.