Guilin Sun
Ansys Employee

I tested a dispersive material but could not duplicate this issue when I used:

setnamed("RCWA","use wavelength spacing",1);

and override the material fitting wavelength range to be the same, for exmple 0.4~0.7um, whereas I tested 0.4~0.5um for 6 points,and 0.4~0.6um for 11 points for a silicon slab with air holes (adpoted from online example the script "rcwa" is depreciated now).

Maybe you can choose visualze the max frequency point and see if they agree.