Search found 407 matches
- 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...
- 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
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
- 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...
- 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.
Should be fine now
PS: You can also report problems like that as github issues.
- 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...
- 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...
- 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
I just fixed the bug in the most recent version on github.
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.
- 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...
- 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
- 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" ...
- 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 ...
- 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 ...
- 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 ...
- 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...
- 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...
- 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.
- 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...
- 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
Anyways, I fixed it (and some other issue it had), it should now work again in the newest version.
Anyways, I fixed it (and some other issue it had), it should now work again in the newest version.
- 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...
- 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...
- 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 ...
- 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...
- 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"...
- 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...
- 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...