Dear ALPS developers and users,

I am using the DMRG algorithm to check the entropic properties of the ground state of an Heisenberg chain. More specifically in the region Jx=Jy=1 and Jz>1 , which is a gapped region. I sweep until I see that the entanglement entropy repeats on a certain pattern through next sweeps. At that stage the simulation should have converged on a good approximation of the ground state, and I take my data. For systems close to the critical point (e.g. Jz=1.001) the results I get are comparable to exact results, and everything works perfectly. When I increase the gap though, I manage to get sensible results only on very short chains.

To explain you what happens on longer chains I am attaching to this mail a plot of the entropy against the iterations. This picture refers to

Jx=Jy=1 Jz=2.5 L=42 Nstates=400 Nsweeps=40 periodic boundary conditions

As you can see it struggles to converge to a sensible value. This picture isa goodrepresentative of any simulation I run. There are two features I see:

1- jumps from a lower value to an higher one: I think this could be explained with the fact that as the system sweeps, it gets short range correlations and it converges immediately to another ground state.

2- the slow convergence in the final stage to a sensible value. This I really don't know how to explain.

Do you know what is going on, and how could I enhance the simulation? Just increasing the number of sweeps or there is a smarter way in your opinion? (It looks like increasing the states kept doesn't change too much the situation, and I really need periodic boundaries for the kind of plot I want to produce).

Many thanks again for your precious help.

Kindest regards.