A method of calculating electromagnetic scattering properties of a structure represented as a nominal structure and a structural perturbation, has the steps: 1008 numerically solving a volume integral equation comprising a nominal linear system 1004 to determine a nominal vector field being independent with respect to the structural perturbation; 1010 using a perturbed linear system 1006 to determine an approximation of a vector field perturbation arising from the structural perturbation, by solving a volume integral equation or an adjoint linear system. Matrix-vector multiplication of a nominal linear system matrix convolution operator may be restricted to sub-matrices; and 1012 calculating electromagnetic scattering properties of the structure using the determined nominal vector field and the determined approximation of the vector field perturbation.