Search found 407 matches

by Johannes
03 Dec 2019, 21:31
Forum: HowTos and FAQ for TeNPy
Topic: Constructing a product state of rotated spins
Replies: 5
Views: 5036

Re: Constructing a product state of rotated spins

Yes, that is indeed what you want. Just for reference, here is the documentation: from_product_state . If you look at the example, one line below the state is normalized, such that it gets np.array([1/sqrt(2), -1/sqrt(2)]) . This is the negative-x state of a spin-1/2 and equivalent to the np.array([...
by Johannes
26 Nov 2019, 19:27
Forum: HowTos and FAQ for TeNPy
Topic: N_sweeps_check & unit cell in infinite dmrg
Replies: 3
Views: 4117

Re: N_sweeps_check & unit cell in infinite dmrg

So, is it fine to miss S? or is it typo? That should be fine, it's just the difference between checking the absolute and the relative error. For the energy, we need to check the relative error: the energy scale can vary by quite a bit simply by rescaling your Hamiltonian. However, the entropy can n...
by Johannes
25 Nov 2019, 17:29
Forum: HowTos and FAQ for TeNPy
Topic: XXZChain antiferromagnet showing no entanglement?
Replies: 1
Views: 3058

Re: XXZChain antiferromagnet showing no entanglement?

The all-up state is an exact eigenstate of the model and hence won't evolve in time, in particular if you use Sz-conservation (which you do in this case): the all-up state is the only state in the sector with S_{total} = L/2 . Try starting from a different state, e.g. all up but one spin down (which...
by Johannes
14 Nov 2019, 10:13
Forum: HowTos and FAQ for TeNPy
Topic: Real-time evolution of the GS of the U=0 Fermi-Hubbard model
Replies: 5
Views: 6507

Re: Real-time evolution of the GS of the U=0 Fermi-Hubbard model

Yes, that's for sure a good idea. Indeed, I thought about it and started to implement it already a few days ago, but I'm quite busy these days (moving to UC Berkeley :)), so I didn't have time to finish it. Hopefully, I can find a bit time next week - unless you volunteer to time to implement it ;-)
by Johannes
11 Nov 2019, 16:03
Forum: Algorithms
Topic: Canonical form for very small singular values
Replies: 1
Views: 8691

Re: Canonical form for very small singular values

One alternative is to iteratively do QR decompositions (and RQ decompositions). You decompose your state as M M M M M = Q R M M M M = Q Q R M M M = ... = Q Q Q Q R and as M M M M M = M M M M R' Q' = M M M R' Q' Q' = ... = R' Q' Q' Q' Q' until your iterations converge. Then your state is in the form ...
by Johannes
11 Nov 2019, 15:50
Forum: HowTos and FAQ for TeNPy
Topic: Real-time evolution of the GS of the U=0 Fermi-Hubbard model
Replies: 5
Views: 6507

Re: Real-time evolution of the GS of the U=0 Fermi-Hubbard model

What order (second or fourth) and timestep do you use for TEBD? Does this go away if you simply use a smaller timestep/higher order? Did you try to look at the overlap with the initial ground state returned by DMRG (as a function of time)? Converging just 3 digits in energy still a bit unprecise, ma...
by Johannes
30 Oct 2019, 17:22
Forum: HowTos and FAQ for TeNPy
Topic: Entanglement entropy for disconnected subsystems
Replies: 4
Views: 4629

Re: Entanglement entropy for disconnected subsystems

Sorry, that was a bug in MPS.get_rho_segment. It should be fixed with the latest version.
by Johannes
29 Oct 2019, 16:45
Forum: HowTos and FAQ for TeNPy
Topic: N_sweeps_check & unit cell in infinite dmrg
Replies: 3
Views: 4117

Re: N_sweeps_check & unit cell in infinite dmrg

In general, the parameter N_sweeps_check should not change the convergence properties - at least not much.... Let's look at what it influences: For checking the convergence, DMRG checks the average change of the energy / entropy per sweep in the last N_sweeps_check steps. The parameter update_env de...
by Johannes
29 Oct 2019, 16:32
Forum: HowTos and FAQ for TeNPy
Topic: Entanglement entropy for disconnected subsystems
Replies: 4
Views: 4629

Re: Entanglement entropy for disconnected subsystems

The function entanglement_entropy_segment should accept disconnected segments, e.g. segment=[0, 2, 4, 6, 8] . However, it works only for *small* enough number of sites in your density matrix - it's exponentially expensive, because it constructs the full density matrix, tracing out the rest of the si...
by Johannes
29 Oct 2019, 16:04
Forum: HowTos and FAQ for TeNPy
Topic: Adding a site to a chain perpendicularly
Replies: 6
Views: 5970

Re: Adding a site to a chain perpendicularly

Hi, indeed, the irregular lattice is still an open 77 . Sorry, there's so much stuff to do. I had to write and defend my PHD thesis, so I didn't have too much time to work on TeNPy lately. Hopefully, this will change again. ;) To make sure I understand correctly, you want a geometry like this: Boson...
by Johannes
15 Oct 2019, 15:15
Forum: HowTos and FAQ for TeNPy
Topic: finite open, finite periodic and infinite periodic chains
Replies: 10
Views: 9141

Re: finite open, finite periodic and infinite periodic chains

is there a way to have a finite periodic chain (so a ring) for the BoseHubbard model? Kind of. We do not support finite MPS with periodic boundaries for the MPS itself i.e. writing | \Psi\rangle = \sum Tr(A^{\sigma_1} \cdots A^{\sigma_L}) | \sigma_1 \cdots \sigma_L\rangle This is a form which is us...
by Johannes
30 Sep 2019, 09:51
Forum: HowTos and FAQ for TeNPy
Topic: print msg into file
Replies: 6
Views: 5956

Re: print msg into file

I've added the `flush=True` to the print statements during the loops of TEBD and DMRG in ce32fd1fc30073918371717beea110083bab8cd1 On my workstation, the flush causes the output to appear immediately. If you redirect to a file on a network-shared file system, there might be a delay of the change bein...
by Johannes
27 Sep 2019, 07:22
Forum: HowTos and FAQ for TeNPy
Topic: About four-site coupling terms
Replies: 2
Views: 3201

Re: About four-site coupling terms

Welcome! There is a way to add such couplings, namly with add_multi_coupling of the tenpy.models.model.MultiCouplingModel instead of just the add_coupling of the tenpy.models.model.CouplingModel . The tenpy.models.toric_code.ToricCode is an example where this is used. Take a look at this topic for m...
by Johannes
26 Sep 2019, 13:02
Forum: HowTos and FAQ for TeNPy
Topic: print msg into file
Replies: 6
Views: 5956

Re: print msg into file

There's also a

Code: Select all

flush=True
option to print().
by Johannes
26 Sep 2019, 12:44
Forum: HowTos and FAQ for TeNPy
Topic: print msg into file
Replies: 6
Views: 5956

Re: print msg into file

Still, there are ways around this. You're describing the problem that the file is "buffered", which Linux/Python does automatically to avoid unnecessary hard disk usage slowing down the execution of the program. However, this can be switched off. For python, try to use the -u option for th...
by Johannes
20 Sep 2019, 14:02
Forum: HowTos and FAQ for TeNPy
Topic: correlation length in idmrg
Replies: 1
Views: 2807

Re: correlation length in idmrg

A such enormous correlation length \xi > 10^10 means that the second eigenvalue of the transfer matrix is 1 up to numerical errors. The reason is that the state you get is a cat state, i.e. a superposition of all |+x \rangle and all |-x \rangle states. See also the discussion in viewtopic.php?f=7&am...
by Johannes
13 Sep 2019, 15:30
Forum: HowTos and FAQ for TeNPy
Topic: iDMRG is not working properly with next-nearest neighbor complex hopping in the Haldane model
Replies: 6
Views: 6673

Re: iDMRG is not working properly with next-nearest neighbor complex hopping in the Haldane model

Welcome Dhiman, and sorry for not answering earlier. You might want to take a look at the tenpy.models.haldane.FermionicHaldaneModel and the tenpy.models.haldane.BosonicHaldaneModel . Both are based on the Hamiltonian from the paper 1407.6985 discussing the fermionic model. As far as I see from a qu...
by Johannes
13 Sep 2019, 14:10
Forum: HowTos and FAQ for TeNPy
Topic: how to write a three-site interaction?
Replies: 4
Views: 31452

Re: how to write a three-site interaction?

Thanks for the clear answer, Umberto :)

Just as little side node: lambda is a special keyword in python.
The usual convention is to append an underscore in such a case, i.e., use lambda_ instead as a variable name ;-)
by Johannes
13 Sep 2019, 14:05
Forum: HowTos and FAQ for TeNPy
Topic: print msg into file
Replies: 6
Views: 5956

Re: print msg into file

There are different standard ways to do this (On all computing clusters I know, there is at least an option to do it automatically). Let me point out a few which should suffice your needs: If you start your program from a Linux console, it's very easy: you can use the > at the and of any command to ...
by Johannes
13 Sep 2019, 13:15
Forum: Implementations
Topic: TEBD for time-dependent Hamiltonians
Replies: 2
Views: 10860

Re: TEBD for time-dependent Hamiltonians

Looks like a good thing. Actually, if you update your local repository, you'll find that the code for the FermiHubbardModel has been moved to tenpy.models.hubbard.FermiHubbardModel . There it says for u1, u2, dx in self.lat.nearest_neighbors: self.add_coupling(-t, u1, 'Cdu', u2, 'Cu', dx) self.add_c...
by Johannes
07 Aug 2019, 09:33
Forum: HowTos and FAQ for TeNPy
Topic: How to assign different hopping parameter 't' to different sites
Replies: 1
Views: 2843

Re: How to assign different hopping parameter 't' to different sites

You don't need to modify the unit_cell , if the kind of sites (Fermions with or without spin, Bosons, spins,...) doesn't change. Instead, you can give a hopping parameter t which is not just a single number, but an array, specifying at which site you want hopping with which strength. Take a look at ...
by Johannes
29 Jul 2019, 13:35
Forum: HowTos and FAQ for TeNPy
Topic: Error when using kroneckerproduct option for GroupedSite
Replies: 5
Views: 4801

Re: Error when using kroneckerproduct option for GroupedSite

I've just added a the function from_MPOModel, which has an example how to use it.
Does that help?
by Johannes
29 Jul 2019, 09:17
Forum: HowTos and FAQ for TeNPy
Topic: periodic conditions
Replies: 7
Views: 5935

Re: periodic conditions

Could it be that the tenpy you are importing is an old version?
The all_onsite_terms was only introduced fairly recently.
by Johannes
26 Jul 2019, 12:27
Forum: HowTos and FAQ for TeNPy
Topic: Error when using kroneckerproduct option for GroupedSite
Replies: 5
Views: 4801

Re: Error when using kroneckerproduct option for GroupedSite

Hi Jack, welcome to TeNPy! (the sites are hardcore bosons, which I've created by simply removing the JW entries in the FermionSite class, I hope this is correct?) The more straightforward way for HardcodeBosons would be to use the tenpy.networks.site.BosonSite with the default Nmax=1 . Here is the i...
by Johannes
08 Jul 2019, 12:26
Forum: HowTos and FAQ for TeNPy
Topic: Finite Chain with different unit cell at the beginning
Replies: 4
Views: 5261

Re: Finite Chain with different unit cell at the beginning

I've opened Issue #77 to keep track of this task, in case you or someone else wants to give it a try ;)