1. A computer-executed method for determining a 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 which includes: an optical component; and a set of mask-corner-rounding (MCR) components, wherein each MCR component is designed to model MCR effects; identifying a set of corners in the mask layout; determining a set of mask layers which includes: a first mask layer which includes substantially all patterns in the mask layout; and a set of MCR mask layers, wherein substantially all patterns in each MCR mask layer relate to patterns in the mask layout that are in proximity to the set of corners, and wherein each MCR mask layer is associated with an MCR component; and using one or more computers to calibrate the uncalibrated process model using the set of mask layers and the process data.