Thanks for the explanation. I have tried and successfully run the Arnoldi to find the dominant eigenvector of a simple random ITensor with (i,j,k...) and (i',j',k'...) but I run into an error when I deal with the transfer matrix which is an ITensor pulled from the ground state MPS (with quantum number). I need to change the "Link" index of it in order to look like (i,j) and (i',j') to use Arnoldi, so I use the replaceInds(ITensor T, IndexSet is1, IndexSet is2) function:
auto lf = leftLinkIndex(psi, XX - Ny + 1);
auto rt = rightLinkIndex(psi, XX);
and the error is "Diagonal elements of QDiag ITensor would have inconsistent divergence". I know I could also replace the index by the delta function
wf *= dag(delta(rt, prime(lf)));
but unfortunately that gives out the same error. I thought the diagonal tensor made from delta() should always have divergence of 0 because it only has non-zero diagonal elements. Why would it have inconsistent divergence with the original ITensor?