On 18 Nov 2013, at 21:46, Rafał arymanus@gmail.com wrote:
Dear Matthias,
Thank you for your reply. I have one more question about DMRG.
I try to calculate a gap of xxz spin model (Jz != Jxy) for different values of Jz and Jxy. I think that it will be more safe to assume that I do not know in what Sz sector search for a ground and excited states.
Is it in this case better option to run simulation over all possible Sz by hand or to comment a line #'Sz_total' : Sz. which should force alps to work in the grand canonical as it was told in the first tutorial on the webpage?
I tried this second approach for the ising model (Jxy = 0, Jz = 1) but what I get was just a duplicated ground state without a correct excited state.
That is because there are two ground states.
What you want to do is to instead run with Sz_total=0 and Sz_total=1 since by spin gap we usually mean the gap between the singlet and triplet sector.