## Search found 429 matches

- 13 May 2024, 05:25
- Forum: HowTos and FAQ for TeNPy
- Topic: How to use MPS.from_Bflat
- Replies:
**1** - Views:
**22**

### Re: How to use MPS.from_Bflat

The finite mps in Tenpy always has open boundary conditions. In other words, in finite systems, the vL on the first tensor should be trivial of dimension 1, and so should be the vR leg of the last tensor. Closing the loop on the virtual legs with a trace is nasty for numerical simulations, since it ...

- 08 May 2024, 14:15
- Forum: HowTos and FAQ for TeNPy
- Topic: Implementing iDMRG for ladder lattices
- Replies:
**2** - Views:
**94**

### Re: Implementing iDMRG for ladder lattices

Let me be a bit more explicit: in the simple case of the Ladder, the following lines in init_couplings are equivalent: add_coupling(-t, 0, 'Cdu', 0, 'Cu', dx=1, plus_hc=True) add_coupling(-tv, 0, 'Cdu', 1, 'Cu', dx=1, plus_hc=True) add_coupling(-t, 1, 'Cdu', 1, 'Cu', dx=1, plus_hc=True) for x in ran...

- 08 May 2024, 14:04
- Forum: HowTos and FAQ for TeNPy
- Topic: Implementing iDMRG for ladder lattices
- Replies:
**2** - Views:
**94**

### Re: Implementing iDMRG for ladder lattices

Hi Fabio, whether you call add_coupling or explicitly loop over add_local_term doesn't matter for the final DMRG run, if you get the same couplings in the MPO of the hamiltonian - but with the latter, you need to iterate over the starting point of the couplings in real space, so you are responsible ...

- 07 May 2024, 09:53
- Forum: Algorithms
- Topic: Gate-based TEBD
- Replies:
**1** - Views:
**219**

### Re: Gate-based TEBD

It doesn't allow this out of the box, but you can easily write your own subclass of the TEBD Engine which just plugs in the corresponding Us. Essentially, you want to re-use the `update_step` function for the evolution (and maybe the first order suzuki trotter decomposition, depending on how you wan...

- 07 May 2024, 08:11
- Forum: HowTos and FAQ for TeNPy
- Topic: Issue with Missing Subscripts on Fermionic Operators in all_coupling_terms().to_TermList() Output
- Replies:
**4** - Views:
**126**

### Re: Issue with Missing Subscripts on Fermionic Operators in all_coupling_terms().to_TermList() Output

I've quickly fixed that formatting to include brackets in 9739ac3a4e26cbbfada009ec6bef6c3d7286a0a8

- 07 May 2024, 07:57
- Forum: HowTos and FAQ for TeNPy
- Topic: Issue with Missing Subscripts on Fermionic Operators in all_coupling_terms().to_TermList() Output
- Replies:
**4** - Views:
**126**

### Re: Issue with Missing Subscripts on Fermionic Operators in all_coupling_terms().to_TermList() Output

The subscript notation is a bit misleading here, because it suggests that it only applies to the next operator. Really, you should read those operators to be grouped like this: 1.00000 * (Cdd Cu)_0 (Cdu Cd)_1 + 1.00000 * (Cdd Cu)_1 (Cdu Cd)_2 + 1.00000 * (Cdd Cu)_2 (Cdu Cd)_3 + 1.00000 * (Cdd Cu)_3 ...

- 07 May 2024, 07:43
- Forum: HowTos and FAQ for TeNPy
- Topic: Applying different time evolution to different subsystems with TEBD
- Replies:
**2** - Views:
**24871**

### Re: Applying different time evolution to different subsystems with TEBD

Jakob is totally right, of course. Let me just mention that it wouldn't be too hard to extend TEBD to work with this. Define a new LadderTEBDEngine, which takes the two models as input, and define 2 separate TEBD engines for them. Then overwrite: def evolve(...): # start with site order # A0 B0 A1 B...

- 16 Apr 2024, 10:15
- Forum: HowTos and FAQ for TeNPy
- Topic: Transfer matrix is incorrect without imposing canonical form
- Replies:
**1** - Views:
**13829**

### Re: Transfer matrix is incorrect without imposing canonical form

If you need to_matrix, why don't you implement that function directly on the existing transfer matrix? To get a consistent result, all tensor which you extract *need* to be in the same canonical form, e.g. all right-canonical 'B' form. After a plain DMRG run (without a call of psi.canonical_form() i...

- 10 Apr 2024, 17:42
- Forum: HowTos and FAQ for TeNPy
- Topic: How can I construct a canonical infinite-temperature state with fixed total number of fermions ?
- Replies:
**1** - Views:
**51157**

### Re: How can I construct a canonical infinite-temperature state with fixed total number of fermions ?

I guess the answer is a bit late, but for those finding this in the future: it should just work like this: In [10]: M = tenpy.FermiHubbardModel(dict(L=10, cons_Sz='Sz', cons_N='N')) In [11]: M.lat.unit_cell[0].leg.chinfo Out[11]: ChargeInfo([1, 1], ['N', '2*Sz']) In [12]: psi = tenpy.PurificationMPS...

- 10 Apr 2024, 17:29
- Forum: HowTos and FAQ for TeNPy
- Topic: Three-site fermionic operator in systems of spinless fermions
- Replies:
**1** - Views:
**10357**

### Re: Three-site fermionic operator in systems of spinless fermions

The error comes because you try to add a term with 3 fermionic operators - this would require an open Jordan wigner string when mapping to spins, and thus be a non-local operator. More generally, you can't write physical Hamiltonians with an odd number of fermionic operators, because they violate lo...

- 10 Apr 2024, 17:04
- Forum: HowTos and FAQ for TeNPy
- Topic: The EE is different for different initial setup
- Replies:
**1** - Views:
**49727**

### Re: The EE is different for different initial setup

This is equivalent to the Su-Schrieffer-Heger (SSH) chain, which has a topolgical phase with edge modes. Given that your filename starts with ssh_, I assume you're familiar with that. the XXZChan has Sz conservation by default. That means if you fix the initial state to be in a particular Sz charge ...

- 10 Apr 2024, 16:50
- Forum: HowTos and FAQ for TeNPy
- Topic: Asimmetry in trivial 2D expansion using TDVP
- Replies:
**2** - Views:
**38862**

### Re: Asimmetry in trivial 2D expansion using TDVP

This reminds me of one of my first work on tensor networks, 1509.00696 - using back then new W_II method that we now have in the ExpMPOEvolution in TeNPy :-) While the Hamiltonian is symmetric under exchange of x/y, the MPS snaking through the square lattice explicitly breaks it - hopping in y-direc...

- 10 Apr 2024, 16:16
- Forum: HowTos and FAQ for TeNPy
- Topic: Strange timing data for DMRG applied to Fermi-Hubbard model
- Replies:
**1** - Views:
**10231**

### Re: Strange timing data for DMRG applied to Fermi-Hubbard model

To reliably benchmark DMRG, I recommend to pin the number of sweeps by setting min_sweeps=max_sweeps to some fixed number - the total run time is just proportional to the number of sweeps, and how many sweeps it does can sometimes depend on subtle differences in convergence thresholds. Indeed, your ...

- 10 Apr 2024, 15:42
- Forum: HowTos and FAQ for TeNPy
- Topic: How to improve calculation accuracy in real time evolution ?
- Replies:
**1** - Views:
**10154**

### Re: How to improve calculation accuracy in real time evolution ?

Tenpy also tries to solve the schrlödinger equation... I guess you compared to exact evolution for a non-interacting system, or exact diagonalization? TDVP does the time evolution with the effective Hamiltonian, i.e. H projected onto the local single-site/two-site MPO. If you have only bond dimensio...

- 10 Apr 2024, 15:17
- Forum: HowTos and FAQ for TeNPy
- Topic: Compressing an MPO
- Replies:
**1** - Views:
**10265**

### Re: Compressing an MPO

At the moment, we don't have this implemented in TeNPy. For finite MPS and small enough systems, you can indeed do this with a simple SVD compression. But for larger and in particular infinite MPS/MPOs, there's some subtleties in doing this, because the MPO is in a Jordan-block form. See 1909.06341 ...

- 10 Apr 2024, 15:11
- Forum: HowTos and FAQ for TeNPy
- Topic: Correlators using iDMRG
- Replies:
**1** - Views:
**10154**

### Re: Correlators using iDMRG

Yes, even if the MPS unit cell is 2, you can have correlations in the MPS way beyond those 2 sites. You can for example call correlation_function with sites2=range(50) to calculate correlations up to site 50, or term_correlation_function_right . And yes, you can also extract the central charge by a ...

- 10 Apr 2024, 15:00
- Forum: HowTos and FAQ for TeNPy
- Topic: Tensor network implementations for transformers and LLMs
- Replies:
**1** - Views:
**10329**

### Re: Tensor network implementations for transformers and LLMs

No. TeNPy is focusing on the simulation of condensed matter physics.

- 10 Apr 2024, 14:58
- Forum: HowTos and FAQ for TeNPy
- Topic: How to add couplings correctly for iDMRG?
- Replies:
**1** - Views:
**10286**

### Re: How to add couplings correctly for iDMRG?

First of all, let me clarify, also for those finding this question later: The system purely 1D / we ignore the y direction, right? I.e. when we talk about periodic boundary conditions, we don't talk about wrapping things on a cylinder or torus or plane, but about a chain (open boundary conditions, O...

- 10 Apr 2024, 14:07
- Forum: HowTos and FAQ for TeNPy
- Topic: Finding ground states of finite systems with fast vanishing gap
- Replies:
**1** - Views:
**10691**

### Re: Finding ground states of finite systems with fast vanishing gap

When you need chi=256 for L=8, you apparently have local hilbert space dimension of 4 - is it a spin-full fermion site? Do you have any charge conservation/symmetries in general? DMRG should usually be able to converge L=32/64 pretty easily. One issue you can have though are close to exact degenerac...

- 10 Apr 2024, 13:14
- Forum: HowTos and FAQ for TeNPy
- Topic: Implementing XXZ model with finite DMRG
- Replies:
**1** - Views:
**49884**

### Re: Implementing XXZ model with finite DMRG

1. Yes - as noted in the comment at the top of the xxz_chain.py implementation. 2. You can always go back in the git history and find it there with git log --all -- examples/xxz_groundstate.py which brings up d1dccf29ee50899451ea95931c5f4216efae8e56 .' 3. For finite, critical 1D systems, the famouse...

- 10 Apr 2024, 12:40
- Forum: Implementations
- Topic: Error bar in the calculated entanglement spectrum
- Replies:
**1** - Views:
**19017**

### Re: Error bar in the calculated entanglement spectrum

Whit error bar you mean "what values do I consider degenerate"? Don't choose that value too small based on what the norm error is: the error in the entanglement spectrum depends drastically on how high the value is. If you plot the full spectrum up to the chi_max, the highest values in the...

- 10 Apr 2024, 12:10
- Forum: Implementations
- Topic: fully connected model
- Replies:
**1** - Views:
**7325**

### Re: fully connected model

Disclaimer: You already said that MPS are not a good idea for this kind of model... That said, I can point you to you misunderstanding: the add_coupling method is for adding a bunch of terms at once, all of the form \sum_i J_{i, i+dx} A_i B_{i+dx} . You want to add a single term A_i B_j , which is w...

- 12 Jan 2024, 11:10
- Forum: Algorithms
- Topic: Choosing dt for time evolve
- Replies:
**1** - Views:
**34224**

### Re: Choosing dt for time evolve

There's no simple, generic rule how large/small dt has to be to ensure convergence, I'm somewhat alarmed by the "periodic boundary conditions". By default, in TeNPy this would just give you one (or ~Ly, if you have a 2D torus) very long-range couplings between the ends of the MPS. Both TDV...

- 12 Jan 2024, 10:34
- Forum: HowTos and FAQ for TeNPy
- Topic: How do I calculate an MPO using an iMPS
- Replies:
**1** - Views:
**23701**

### Re: How do I calculate an MPO using an iMPS

This is ill-defined for infinite systems: The overlap \braket{s | t} for any two iMPS which do not represent exactly the same state goes to 0 in the the thermodynamic limit (or to infinity, if you don't have normalized MPS.). Also, you generally don't want to write down "cat state" superpo...

- 23 Nov 2023, 08:31
- Forum: HowTos and FAQ for TeNPy
- Topic: How to realize SU(2) symmetric DMRG?
- Replies:
**2** - Views:
**28129**

### Re: How to realize SU(2) symmetric DMRG?

You should be able to get to significantly larger system sizes even on desktops. Typical run times range from several hours up to a couple of days (up to one or two weeks) on typical desktops / single-node cluster machines, depending on what's available to you. The usual geometry of choice are long,...