September 2018 - Comp-phys-alps-users

How can I extrapolate a nonlinear curve to D->Inf limit?
by adfws 27 Sep '18

27 Sep '18

Hi everyone, I'm reading DMRG tutorials in http://alps.comp-phys.org/mediawiki/index.php/ALPS_2_Tutorials:DMRG-01_DMRG. and http://alps.comp-phys.org/mediawiki/index.php/ALPS_2_Tutorials:DMRG-02_Gaps It suggested me to extrapolate energies and energy gaps to D(maxstates) -> infinity. The question is about nonlinear extrapolation. In the case spin 1/2 gap versus 1/L, there is a linear relation and I can easily polyfit it linearly and get the gap in L->Inf limit. But in the case of quantities such as gaps versus D(maxstates), there is no simple linear relation. For example: Each time I enlarge D by 50, energy gaps change by the following values: [ -2.49851713e-08, -1.45270462e-11, -2.84217094e-14, -5.50670620e-14] which slows down changing by smaller and smaller values. I have done some exponential curve fittings in matlab but it become confusing when you have many models to fit it. How can I extrapolate such a nonlinear curve to D->Inf limit? Is there any standard way to do this? LiuBiao ll070616(a)126.com

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Why spin 1 chain is added two spin 1/2 sites at its both ends?
by adfws 20 Sep '18

20 Sep '18

Hi everyone, I'm reading DMRG tutorials in http://alps.comp-phys.org/mediawiki/index.php/ALPS_2_Tutorials:DMRG-01_DMRG. When treating spin 1 Heisenberg chain with open boundary conditions, 2 spin 1/2 sites were added to both ends of the spin 1 chain. Why? I have completely no idea about this special treatment. Can anyone give further explanations? LiuBiao ll070616(a)126.com

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Staggered susceptibility in dirloop_sse of a spin chain
by Stanislaw Galeski 07 Sep '18

07 Sep '18

Dear All, We are trying to reproduce results from this paper using the dirloop_sse code. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.89.047202 Typically we setup the simulation such that it calculates the spin-spin correlations and then calculate the staggered susceptibility from that. The results for uniform susceptibility obtained that way seem to reproduce well the known results, however it is not the case of the staggered susceptibility. In particular the staggered susceptibility seems to be the same for even and odd chain lengths and to scale in a bizarre way with length. This happens even for very short chains where we would expect the difference between odd/even to be significant. Is this a known problem with a simple explanation? Regards, Stanislaw and Kirill

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