eigen solver cst

I want to find the bandgap of periodic structure using CST for gama-x-m-gama (2D) diagram. would you please halp me?

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Hi Coungtran
I read all of the topics and comments before. I have done all suggestion but I can not obtain same results as mentioned in the papers (for example rahmat-sammi and sievenpiper). the shape of my curves is similar to those but not the same!
Now what is your suggestion