Current model based OPC software operates under a set of simple guiding principles. First, a design is fragmented into finitely sized segments, the sizes and numbers of which are limited by run-time and mask constraints. Within each fragment the intensity (aerial image) and edge-placement error (EPE) are calculated at a single location. Finally, the length of the entire fragment is moved to correct for the EPE at that location. Although the computation of intensity and EPE are “model based”, the fragmentation and simulation site placement are typically “rules based”. Problems with this methodology can arise whenever the location of the fragments and simulation points are non-optimal. This can be of particular concern with very low-k1 lithography employing hard off-axis illumination, where aerial image “ripples” are a known issue. The authors will propose several solutions to these issues involving model based optimization and placement of fragments and simulation sites.