As the k1 factor of lithography process goes lower, model-based optical proximity correction (OPC) has become the most important step of post-tape-out data preparation for critical mask levels. To apply model-based OPC, a lithographic model with optical and resist parameters usually generated by a regression is required. It takes significant turn-around-time (TAT) to obtain the OPC model, normally more than 1 day per mask level. In this paper, we present an automatic and effective OPC model extraction method using the adaptive simulated annealing (ASA) algorithm. By applying this algorithm to extract the optimal model parameter values, we reduced the model parameter fitting time to less than 1 hour. We confirm the reliability and accuracy of the model generated by this method. With this newly developed automatic modeling method, we present a methodology to detect the critical failure on the wafer effectively that can occur by the focus variation during the lithography process. Generally, we sample only one set of measurement CD data taken under a controlled process condition with the best focus. Based on measurement data at the best focus, the in-house lithography simulator, FAITHTM, can generate simulated CD data for the multiple defocus levels without measurement data at the variable defocus levels. The multiple defocus models are built based on the simulated CD data and the automatic OPC modeling method makes the model buildings very fast. Finally, through the simulation of OPC result according to the multiple defocus models, we can verify or forecast the defocus effect before realistic patterning on wafers efficiently. We show the capability of weak point detection by this methodology on the 80nm DRAM devices with ArF photolithography.