1. A method for optimizing the conditions of illumination of a lithographic apparatus by computer simulation, the lithographic apparatus comprising an illuminator and a projection system, the method comprising: defining a lithographic pattern to be printed on a substrate; selecting a simulation model; selecting a grid of source points in a pupil plane of the illuminator; calculating separate responses for individual source points, each of the responses representing a result of a single or series of simulations using the simulation model; and adjusting an illumination arrangement of the illuminator based on analysis of accumulated results of the separate calculations.