Hello George,
By multilayer PBG are you referring to a multilayer film (i.e. 1D photonic crystal)? If that is the case there is a step by step tutorial starting <“here”>https://optiwave.com/category/optifdtd-manuals/optifdtd-background-and-tutorials/b4-lesson-11-analyzing-1d-photonic-crystals-bragg-gratings/</a>. Please not there is a typo in that less in that the Domain Origin in the PWE parameters dialog box is not the lattice origin plus the A, B, and C offsets, it is just those offsets. Therefore you either need to set C = 2 in the Domain Origin section of the PWE parameters dialog or you need to set you PBG component to start at 0 (and increase the number of periods from 8 to 10) instead of 2.
This tutorial reproduces the right sub-figure in Figure 2 of the first edition of Molding the Flow of Light.
Scott