FDTD will need to fit the dispersion internally. You can improve the fitting by adjust various parameters in the fitting panel, including those in "show advanced". Please try.
You can refer this link https://optics.ansys.com/hc/en-us/articles/360034915053-Tips-for-improving-the-quality-of-optical-material-fits
If you are a premium user, please send us your file through email so we can better help you. You can register for premium support if you have the code:
Please Login to Report Topic
Please Login to Share Feed