In some situations you might find that the material fit for a given material works very well for the real part of the permittivity but it does poorly for the imaginary part. An example is “Ag (Silver) - Palik” with the default settings for the wavelenght range 0.4-1.1 microns:
You might try to solve this problem by reducing the fit tolerance. However, you will find that even for 0.0001, there is no significant change in the material fit:
What parameter should we change then? In this type of situations usually the best approach is to increase the “imaginary weight”, as you can see here:
The reason why this parameter is so useful in this case is because the magnitudes of the real and imaginary parts of the permittivity are quite different, so giving more weight in the fitting to the imaginary part works quite well.