18. The method of claim 17 , wherein the trained reference model comprises a top critical dimension model and a bottom critical dimension model, and determining the correlation metric further comprises: determining a first partial derivative of the top critical dimension model at the expected operating dose and focus point with respect to dose; determining a second partial derivative of the top critical dimension model at the expected operating dose and focus point with respect to focus; determining a third partial derivative of the bottom critical dimension model at the expected operating dose and focus point with respect to dose; determining a fourth partial derivative of the bottom critical dimension model at the expected operating dose and focus point with respect to focus; and generating the correlation metric based on the first, second, third, and fourth partial derivatives.
17. The method of claim 10 , further comprising determining a correlation metric for the trained reference model at an expected operating dose and focus point for the photolithography system.