Reply To: Material is not fitted properly and might be giving problems with convergence

[Guilin Sun]

The fitting mechanism is to ensure the causality. Thus it has special requirement for the fitting. Lumerical multi-coefficient material mode works the best for most natural materials. However in some cases it can be challenge such as your case. 

You may reduce the number of coefficients (as 20 does not work now),  try the advanced fitting feature to use smaller weight factor to enhance the real part fitting. Some tips can be found here:

Tips for improving the quality of optical material fits

Using the Material Explorer to view and adjust optical material models