Good OPC model calibration structures should be representative of and span the dimensions and layout forms that will be found in the product on which the model will be applied. If model fitting is done using edge placement (EPE) methods, only symmetric structures can be used and this constrains the model fitter to a classic but limited set of calibration structures. The most critical features, such as those from a bit cell tend to be asymmetric. While asymmetric structures have typically been used for model verification, using them in model calibration structures provides more degrees of freedom for the calibration test structures to capture two dimensional behavior. This produces more robust, accurate models which yield better quality corrections on wafer. During process development models are re-calibrated as the process is adjusted and optimized. In some cases particularly important critical configurations can be added to the calibration set to insure maximum accuracy on those features. As these configurations are extracted from real designs, they are rarely symmetric. This paper describes how by using a CD-based rather than an edge-placement based modeling approach, OPC models can be created from asymmetric, more product-like type structures, and demonstrates how this can allow better predictability on other verification structures. The paper will also review the two types of model forms commonly used (Constant and Variable Threshold models) and compare their performance while using asymmetric calibration structures.