Dear ALPS users and developers:
I want to study a spin ladder with four-site interaction. [ Phys. Rev. B 67, 100409(R) (2003), http://prb.aps.org/abstract/PRB/v67/i10/e100409 ]
As ALPS 2.0 do not support 4-site terms, I code DMRG myself. For small size system(L=2,4,6,8,10) and the four-site interaction is zero, I have checked my DMRG code by comparing the obtained ground-state energy with that obtained by ED in ALPS. The two are fairly consistent.
Then I move on to larger systems and four-body term are included. However, I am confused by the DMRG results: in the staggered dimer state of the system(see Phys. Rev. B 67, 100409(R) (2003), Fig. 2), when the ground-state of the ladder has converged, I check the two-site correlations, and find that the correlation function [ G_leg ] between nearest-neighboring two sites on the leg converges very well, meanwhile the correlation function [ G_rung ] between the two sites on the same rung does not converge at all ( G_rung seems to be random with the increase of size L, L = 100 ).
I wonder is this a reasonable result ( is it possible that G_rung becomes undefined or because of some other physical reason, leading to a random average value for G_rung ) ? Or all the pairwise correlations in the system should always be converged after the gound-state energy converges ? Any suggestion would be appreciated.
sunzhaoyu2000@gmail.com