A method for determining kernels in a sum of coherent systems (SOCS) approximation is provided. Information for an object to be simulated in a manufacturing process is determined. For example, information based on geometries that are included in a layout or mask is determined. A set of kernels from a transmission cross coefficient (TCC) matrix are also determined. The set of kernels may be weighted by importance values in an order of importance. The kernels may then be re-ordered based on the information for the object. These kernels are then re-ordered in the SOCS series to reflect their order of importance. The SOCS series of kernels is then truncated at the number of kernels desired. Accordingly, by re-ordering the kernels that may be more relevant to the object to include higher weights, when the truncation occurs, the kernels that are most relevant may be included in the SOCS approximation.