A process of calibrating parameters of a stack model used to simulate the performance of measurement structures in a patterning process, the process including: obtaining a stack model used in a simulation of performance of measurement structures; obtaining calibration data indicative of performance of the measurement structures; calibrating parameters of the model by, until a termination condition occurs, repeatedly: simulating performance of the measurement structures with the simulation using a candidate model; approximating the simulation, based on a result of the simulation, with a surrogate function; and selecting a new candidate model based on the approximation.