I am trying to run ALPS DMRG to calculate gap for a spin system on 3-chain lattice. My energy eigenvalue is degenerate and I am trying to divide the system to different sector using the constraint. I want to set an eigenvalue of one operator to have a specific value.

My problem is the operator which divide my Hilbert space to different sector is not local and depends on several site. I am wondering if I can do that with constraint.

Should I define a new quantum number other than Sz?

Thank you for your help,

Best,

Ali