Search found 407 matches

by Johannes
22 Mar 2019, 10:02
Forum: HowTos and FAQ for TeNPy
Topic: How to implement boundary pinning field in iDMRG appropriately?
Replies: 2
Views: 3304

Re: How to implement boundary pinning field in iDMRG appropriately?

In iDMRG, the iMPS unit cell is the central part of a growing environment on the left and right. The idea of the Engine.init_env() is to re-use the environment (and state) from a previous run. If the state has a sufficiently large correlation length, the environmnet "pins" DMRG to find the...
by Johannes
14 Mar 2019, 15:10
Forum: Other News
Topic: Serious bug in models
Replies: 0
Views: 35079

Serious bug in models

Another serious bug, this time in the models.
The problem is/was that adding the hermitian conjugate with was not done correctly.
Further details at issue #23.

:!: If you have implemented your own model, check that you do it correctly in your case :!:
by Johannes
13 Mar 2019, 13:54
Forum: Other News
Topic: Serious bug in npc.inner
Replies: 0
Views: 13615

Serious bug in npc.inner

I found a serious bug in npc.inner . This bug could result in wrong expecation values and Lanczos/DMRG giving completely wrong results. Luckily, the bug was only on the github master branch for two weeks (since March 02), so I hope it didn't cause too many problems yet. In case you did a "git p...
by Johannes
13 Mar 2019, 09:23
Forum: HowTos and FAQ for TeNPy
Topic: Finite DMRG not working after update to new version
Replies: 4
Views: 4427

Re: Finite DMRG not working after update to new version

Sorry, that's indeed true. The bug only appeared if you use a mixer and have a model without onsite terms.
Should be fine now ;)

PS: You can also report problems like that as github issues.
by Johannes
12 Mar 2019, 17:29
Forum: HowTos and FAQ for TeNPy
Topic: Finite DMRG not working after update to new version
Replies: 4
Views: 4427

Re: Finite DMRG not working after update to new version

I recently changed the order of axes in DMRG from (vR.p1) to (p1.vR) to avoid unnecessary transposes. However, the newest version should consistently use "(p1.vR)". Did you actually store some MPS (e.g. with pickle) and load it again? Or do you restart the whole simulation/initialization o...
by Johannes
06 Mar 2019, 10:18
Forum: HowTos and FAQ for TeNPy
Topic: Exponentiation of operator / String order parameter
Replies: 11
Views: 58466

Re: Exponentiation of operator / String order parameter

I just noticed an error in my last post above, see the edit. I hope that was clear anyways...
by Johannes
26 Feb 2019, 11:01
Forum: HowTos and FAQ for TeNPy
Topic: Error drawing Lattice in Tenpy
Replies: 2
Views: 2839

Re: Error drawing Lattice in Tenpy

Thanks for pointing that out.
Luckily, both problems were just a problem of the Lattice.plot_coupling(ax) function, and not the actual couplings of the model.
I just fixed the bug in the most recent version on github.
by Johannes
22 Feb 2019, 09:35
Forum: HowTos and FAQ for TeNPy
Topic: Exponentiation of operator / String order parameter
Replies: 11
Views: 58466

Re: Exponentiation of operator

The np_conserved.expm() expects a np_conserved Array, not a numpy array or string (it doesn't know anything about what you might mean with "Sz"). Therefore, you should get the Sz-operator as an np_conserved.Array, preferably out of your "Site" classes. Assuming that you have a un...
by Johannes
22 Feb 2019, 09:05
Forum: HowTos and FAQ for TeNPy
Topic: Exponentiation of operator / String order parameter
Replies: 11
Views: 58466

Re: Exponentiation of operator

Oh, sorry, I had the Pauli Sz in mind, and the factor 1/2 of course matters. My bad ;)
by Johannes
21 Feb 2019, 16:25
Forum: HowTos and FAQ for TeNPy
Topic: Twisted Boundary Conditions for a CouplingMPOModel
Replies: 3
Views: 4363

Re: Twisted Boundary Conditions for a CouplingMPOModel

Yes, as Leon pointed out, you need to modify the prefactor for the hopping constants, such that an electron picks up a phase when going around the cylinder. This can be done by providing a (numpy) array instead of just a scalar as parameter. Another very important thing here is to "reuse" ...
by Johannes
21 Feb 2019, 13:43
Forum: HowTos and FAQ for TeNPy
Topic: Exponentiation of operator / String order parameter
Replies: 11
Views: 58466

Re: Exponentiation of operator

Since exponentiating a matrix is often needed for tensor networks, the scipy.linalg.expm function is also implemented for np_conserved Arrays . When you talk about string order parameters, do you really want to calculate that? What you wrote suggests that you try to calculate something like \langle ...
by Johannes
18 Feb 2019, 09:34
Forum: HowTos and FAQ for TeNPy
Topic: Interpretation of qnumber>1 for Entanglement Spectra
Replies: 3
Views: 3841

Re: Interpretation of qnumber>1 for Entanglement Spectra

The "charges" you get are the "quantum numbers" you conserved. Of course, it depends on the model which "quantum numbers" you can actually preserve. For example, for a spin-model, you can preserve the Magnetization (e.g. Heisenberg XXZ chain), or just the parity of the ...
by Johannes
06 Feb 2019, 23:44
Forum: HowTos and FAQ for TeNPy
Topic: Investigations of the Hubbard model
Replies: 2
Views: 3946

Re: Investigations of the Hubbard model

The general setup looks fine. Regarding the bond dimension, you need to carefully check if larger \chi leads to different results. I would be very surprised if 30 is enough for convergence. To check that it works you need to compare results you have with existing results in the literature ;) Finite ...
by Johannes
05 Feb 2019, 15:53
Forum: HowTos and FAQ for TeNPy
Topic: perm parameter in compute_K function
Replies: 2
Views: 11289

Re: perm parameter in compute_K function

The correct permutation depends on the lattice you choose, and possibly on the "order" argument of that lattice. Yes, you should simply call it like psi.compute_K(perm=M.lat) # where M is your model to automatically figure out the "correct" permuation. This permutation will corre...
by Johannes
05 Feb 2019, 15:42
Forum: HowTos and FAQ for TeNPy
Topic: How to generate a random MPS in tenpy
Replies: 5
Views: 6778

Re: How to generate a random MPS in tenpy

Sorry for not coming back to this earlier. The real question is: what do you mean with "random" when talking about an MPS? When you draw a "random" state uniformly from the Hilbert space, it has a volume law entanglement; in fact it is almost maximally entangled! Therefore, it wo...
by Johannes
01 Feb 2019, 14:40
Forum: HowTos and FAQ for TeNPy
Topic: Implementing quantum gates
Replies: 2
Views: 2880

Re: Implementing quantum gates

What kind of "gate" do you have in mind? The usual TEBD where the "gates" are U=exp(i H_bond dt) is implemented in TeNPy.
by Johannes
31 Jan 2019, 10:20
Forum: Algorithms
Topic: Some questions about DMRG
Replies: 2
Views: 10300

Re: Some questions about DMRG

On general grounds: DMRG is a variational method which looks for the best approximation of the ground state in the space of MPS with a given maximal bond dimension (assuming you put a "chi_max" in the DMRG_parameters). This class of MPS can only represent states with a entanglement entropy...
by Johannes
25 Jan 2019, 15:12
Forum: HowTos and FAQ for TeNPy
Topic: xxz_corrlength.py example gives error 'out of the box'
Replies: 2
Views: 3217

Re: xxz_corrlength.py example gives error 'out of the box'

The problem was just that pickle needs the files to be opened in "binary" mode since Python 3, and the example stems from a time where TeNPy was still running with Python 2 :D
Anyways, I fixed it (and some other issue it had), it should now work again in the newest version.
by Johannes
21 Jan 2019, 09:32
Forum: HowTos and FAQ for TeNPy
Topic: Expectation values using GroupedSite
Replies: 2
Views: 3028

Re: Expectation values using GroupedSite

psi.expectation_value("Sx") tries to evaluate the expectation value \langle S^x_i\rangle on each site i of the MPS. If you called psi.group_sites(k) beforehand, the local sites i refer to GroupedSite , each containing k of the previous sites, and the "Sx" is no longer defined (o...
by Johannes
17 Jan 2019, 15:26
Forum: HowTos and FAQ for TeNPy
Topic: the truncation in dmrg
Replies: 5
Views: 5575

Re: the truncation in dmrg

That's the point: if your result is not unique (due to the degeneracy), any (small) change of the setup can change the result - it's basically random, what you get. Your results should usually still be reproducable, when you run the same code on the same machine with the same python version, but cha...
by Johannes
17 Jan 2019, 11:26
Forum: HowTos and FAQ for TeNPy
Topic: the truncation in dmrg
Replies: 5
Views: 5575

Re: the truncation in dmrg

The problem you have met comes from the degeneracy of the states - you have a bunch of correct, possible solutions, and which one is returned depends on numerical round off errors. I don't get the comment on truncation. Let me clarify: The truncation respects all the parameters chi_max, svd_min and ...
by Johannes
08 Jan 2019, 13:49
Forum: HowTos and FAQ for TeNPy
Topic: the truncation in dmrg
Replies: 5
Views: 5575

Re: the truncation in dmrg

What's even more worrying is that the expectation values of say "Sz" are also changing :!: However, it's not a bug of TenPy, but there is a simple explanation for this behaviour :roll: The problem is that the ground state is degenerate (up to a finite size gap below the precision). You cho...
by Johannes
18 Dec 2018, 16:37
Forum: HowTos and FAQ for TeNPy
Topic: Unit cell with different types of sites
Replies: 4
Views: 5516

Re: Unit cell with different types of sites

This is actually a common problem that makes very much sense is comes from the fact that DMRG optimizes locally (just two sites at once) and the special initial state - product states are very special, having no entanglement at all... People often say that DMRG is "stuck in a local minima"...
by Johannes
05 Dec 2018, 17:33
Forum: Algorithms
Topic: Infinite DMRG state prediction
Replies: 2
Views: 9952

Re: Infinite DMRG state prediction

Hi Jason, Welcome to the forum :) very good question! In TeNPy, this is a bit hidden. Actually, the "state prediction" is in the line 103 of the toycode: theta0 = np.reshape(self.psi.get_theta2(i), [Heff.shape[0]]) # initial guess This line gets the "initial guess" for the ground...
by Johannes
21 Nov 2018, 20:45
Forum: HowTos and FAQ for TeNPy
Topic: Unit cell with different types of sites
Replies: 4
Views: 5516

Re: Unit cell with different types of sites

That's a very good question, thanks for asking! Each of the different Site classes get their own instance of the ChargeInfo class, but all the sites involved into one tensor network (e.g. a MPS or MPO) need to have the same ChargeInfo. When you don't conserve the boson number, both sites agree by sa...