Search found 407 matches

by Johannes
02 Aug 2023, 15:15
Forum: Algorithms
Topic: How are periodic boundary conditions implemented?
Replies: 3
Views: 11883

Re: How are periodic boundary conditions implemented?

I learned the trick in private communications from Ian McCulloch several years back, but I'm also not sure who was the first to suggest/use it, sor I can't provide a reference...
by Johannes
17 Jul 2023, 16:57
Forum: HowTos and FAQ for TeNPy
Topic: Calculating Time Evolution Only Once
Replies: 4
Views: 7703

Re: Calculating Time Evolution Only Once

I'd always try to exploit "trivial" parallelization first, where you can just distribute jobs (say for different Hamiltonian parameters, or in your case different initial states) to different nodes on a cluster. If you can't do that because you're working on a local machine, keep it as ind...
by Johannes
17 Jul 2023, 16:31
Forum: HowTos and FAQ for TeNPy
Topic: DMRG on 2D triangular lattice for J1-J2 spin-1/2 model
Replies: 1
Views: 4838

Re: DMRG on 2D triangular lattice for J1-J2 spin-1/2 model

You need to be aware of symmetries in your system. The SU(2)-symmetric heisenberg interactions in particular preserve the total Sz spin, and the ferromagnetic all-up state (a) in your picture is an exact eigenstate. Hence, when you start from this state, DMRG just doens't update it - that's why your...
by Johannes
17 Jul 2023, 10:14
Forum: HowTos and FAQ for TeNPy
Topic: Calculating Time Evolution Only Once
Replies: 4
Views: 7703

Re: Calculating Time Evolution Only Once

I'm not sure if I understand your question correctly - do you suggest to write U(t) as an MPO? Unfortunately, it doesn't work that way. Writing down the full time evolution operator (say as an MPO) for large t is much more expensive than just applying it to a given state: it requires exponentiating ...
by Johannes
17 Jul 2023, 08:41
Forum: Implementations
Topic: Defining a New Operator when Inheriting CouplingMPOModel
Replies: 1
Views: 8047

Re: Defining a New Operator when Inheriting CouplingMPOModel

The issue here is that you directly pass the full Ladder instance in the model_params, which already has the site defined - hence the model doesn't call init_sites anymore. Try to use model_params = dict( lattice='Ladder', L=20, U=1., bc_MPS='finite' ) and overwrite init_lattice if you need to adjus...
by Johannes
20 Jun 2023, 18:26
Forum: HowTos and FAQ for TeNPy
Topic: Save evolution with hdf5
Replies: 1
Views: 1442

Re: Save evolution with hdf5

First of all, I would in general advise you to think about the extra amount of storage you need for this, and whether this is sensible, of whether it's easier to re-run the time evolution when you decide that you really need extra measurements. At least think about whether you want to save it at *ev...
by Johannes
20 Jun 2023, 10:09
Forum: HowTos and FAQ for TeNPy
Topic: state overlap(probability)
Replies: 11
Views: 7649

Re: state overlap(probability)

Just 'up'. I forgot to copy over corresponding line in the example, see the edit above ;-)
You can also put the index directly, if you know it - but be aware of the sorting of charges....
by Johannes
20 Jun 2023, 10:07
Forum: HowTos and FAQ for TeNPy
Topic: Infinite DMRG with conserved charges
Replies: 2
Views: 2086

Re: Infinite DMRG with conserved charges

Just to make sure I understand: when you say [b_i,b^\dagger_j]=i\delta_{ij} , was the i supposed to be there??? I guess not, correct? In the Hamiltonians you wrote, each b is acting on the same site j - I think you mean j, j+1 and j+2 (for 3 sites), correct? That said, in your models, you don't coup...
by Johannes
20 Jun 2023, 09:09
Forum: HowTos and FAQ for TeNPy
Topic: Question about poorly conditioned H matrix in KrylovBased
Replies: 6
Views: 1840

Re: Question about poorly conditioned H matrix in KrylovBased

1) What model are you actually using? The site you define has Sz conservation, which you can't use in the Toric code. 2) What intitial state are you using? Is H psi = 0 on that initial state? 3) What's the log output during the DMRG run? 4) Again, are your individual entanglement entropies larger th...
by Johannes
20 Jun 2023, 08:48
Forum: HowTos and FAQ for TeNPy
Topic: state overlap(probability)
Replies: 11
Views: 7649

Re: state overlap(probability)

at a single site only? Then I would suggest to just use apply_local_op with a local operator |x><x|, where x is the state you want to project on. def project_local(psi, i, state): site = psi.sites[i] P = np.zeros(site.dim) P[site.state_index(state)] = 1. proj = npc.diag(P, site.leg, labels=['p', 'p*...
by Johannes
16 Jun 2023, 09:04
Forum: HowTos and FAQ for TeNPy
Topic: How to add the resume from checkpoint in yaml file
Replies: 2
Views: 1156

Re: How to add the resume from checkpoint in yaml file

At the moment, you need to directly call the tenpy.resume_from_checkpoint function with the filename from where you want to resume the simulation. You can do this from the command line e.g. with python -c 'import tenpy; tenpy.resume_from_run("path_to_file.h5")' . See 269 for ToDo / progres...
by Johannes
16 Jun 2023, 08:37
Forum: HowTos and FAQ for TeNPy
Topic: How to obtain a more accurate charge-pumping in tenpy
Replies: 1
Views: 1531

Re: How to obtain a more accurate charge-pumping in tenpy

The chi_list forces DMRG to project to small bond dimension initially and the increase chi as specified with increasing number of sweeps. The idea is to gradually get close to the true ground state. When doing the charge pump, you want to adiabatically follow the ground state with as little change a...
by Johannes
16 Jun 2023, 08:16
Forum: HowTos and FAQ for TeNPy
Topic: Question about poorly conditioned H matrix in KrylovBased
Replies: 6
Views: 1840

Re: Question about poorly conditioned H matrix in KrylovBased

For you model: double-check that your hamiltonian is indeed hermitian assert model.H_MPO.is_equal(model.H_MPO.dagger()) - if you mess up adding the couplings, it might not be. To fully avoid the warning, you might need to start from a different initial state. In particular, I'd assume you start from...
by Johannes
01 Jun 2023, 15:20
Forum: HowTos and FAQ for TeNPy
Topic: How to adjust the 'N_sweeps_check' and 'update_env' parameters in Tenpy's idmrg calculation
Replies: 2
Views: 1942

Re: How to adjust the 'N_sweeps_check' and 'update_env' parameters in Tenpy's idmrg calculation

the N_sweeps_check defines how often you check for convergence and print the status output to the log. Many iDMRG runs need >100 sweeps to converge, but every sweep is much faster than comparable finite-size DMRG runs (since L_x_infinite ~1-2 << L_x_fininte ~ 50=200 for typical runs). Assuming that ...
by Johannes
31 May 2023, 09:20
Forum: HowTos and FAQ for TeNPy
Topic: Question about poorly conditioned H matrix in KrylovBased
Replies: 6
Views: 1840

Re: Question about poorly conditioned H matrix in KrylovBased

The "poorly conditioned H" is not an error message, it's a warning - telling you that there might be something wrong in your setup. The poorly conditioned H is often a hint for 1) your Hamiltonian not being hermitian or 2) the initial state/ some intermediate state being such that H |psi> ...
by Johannes
24 May 2023, 15:17
Forum: HowTos and FAQ for TeNPy
Topic: Non-canonical MPS as an initial guess for DMRG
Replies: 2
Views: 1077

Re: Non-canonical MPS as an initial guess for DMRG

For finite DMRG, it doesn't really matter, as it effectively brings the MPS to canonical form in the first sweep. For infinite DMRG, I'm not completely sure right now, if it's okay. The subtle thing there is that you absorb the tensors into the environments, which you grow each sweep. The MPO assume...
by Johannes
23 May 2023, 14:32
Forum: HowTos and FAQ for TeNPy
Topic: Translationally invariant simulation of XXZ
Replies: 1
Views: 1050

Re: Translationally invariant simulation of XXZ

When you have a perfect ground state degeneracy, DMRG is by definition allowed to give you any superposition (of the ground states) it wants, and it tends to break the translation invariance if it can. The state you wrote down is what we call a "cat" state, because it is a global superposi...
by Johannes
23 May 2023, 13:57
Forum: HowTos and FAQ for TeNPy
Topic: how to write HofstadterModel with mixed_xk in infinite dmrg
Replies: 7
Views: 4301

Re: how to write HofstadterModel with mixed_xk in infinite dmrg

In real space, you can't conserve ky directly, so you don't get that quantum number (although it's still there in principle, yes). What initial state did you try? You need to put in the correct number of particles, but the total ky is a free parameter which you might need to vary as well (if you don...
by Johannes
23 May 2023, 13:41
Forum: HowTos and FAQ for TeNPy
Topic: exponential of the sum of operators on different sites
Replies: 3
Views: 1229

Re: exponential of the sum of operators on different sites

Okay, it wasn't clear to me that r < Lx, such that this is a non-trivial operator (given charge conservation). In the paper, they use r = L/2, what did you try? if you always start at x=0, it's actually just measuring the parity of particles to the left at a given bond (say in the center at L/2, but...
by Johannes
22 May 2023, 17:05
Forum: HowTos and FAQ for TeNPy
Topic: exponential of the sum of operators on different sites
Replies: 3
Views: 1229

Re: exponential of the sum of operators on different sites

I'm confused what the problem is. Clearly, there's a mathematical identity \exp(i \pi \sum_y( n_{x,y} - 1)) = \prod_y \exp(i \pi (n_{x,y} - 1)) , so \prod_x \exp(i \pi \sum_y( n_{x,y} - 1)) = \prod_{x,y} \exp(i \pi (n_{x,y} - 1)) , which is a product of exponentials of onsite-operators. How do you k...
by Johannes
22 May 2023, 16:58
Forum: HowTos and FAQ for TeNPy
Topic: How to create a new operator as a combination of operators acting on different sites
Replies: 2
Views: 1239

Re: How to create a new operator as a combination of operators acting on different sites

Welcome! Unfortunately, we don't support multi-site operator combinations in TeNPy very well at the moment. However, there's a very simple, general work-around: just write everything explicitly in terms of the single-site operators and sum it up afterwards. In your case, just plugging in \kappa^z_j ...
by Johannes
22 May 2023, 16:01
Forum: HowTos and FAQ for TeNPy
Topic: how to write HofstadterModel with mixed_xk in infinite dmrg
Replies: 7
Views: 4301

Re: how to write HofstadterModel with mixed_xk in infinite dmrg

Hi Kevin, just like the other 2D models in TeNPy, the `MixedXK` model reads out the Lx parameter to define the number of "rings" in the iMPS unit cell, similar as Ly defines the number of sites / lattice unit cells around the cylinder. The add_onsite and add_coupling methods accept the str...
by Johannes
08 May 2023, 10:37
Forum: HowTos and FAQ for TeNPy
Topic: how to write HofstadterModel with mixed_xk in infinite dmrg
Replies: 7
Views: 4301

Re: how to write HofstadterModel with mixed_xk in infinite dmrg

The Hofstadter models in TeNPy are implemented in real-space (both along and around the cylinder). To switch to momentum space, you might want to take a look at the tenpy.models.mixed_xk.MixedXKModel , and the subclass examples tenpy.models.mixed_xk.SpinlessMixedXKSquare and tenpy.models.mixed_xk.Hu...
by Johannes
27 Apr 2023, 07:55
Forum: HowTos and FAQ for TeNPy
Topic: Behaviour of lattice.find_coupling_pairs()
Replies: 1
Views: 941

Re: Behaviour of lattice.find_coupling_pairs()

Maybe the misleading thing is that we provide default parameter for cutoff. This function is supposed to help in case you define a new Lattice and want to find some useful pairs to define. Of course, finding the nearest- and next-nearest neighbor pairs is still quite easy, but when you reach fourth-...
by Johannes
24 Apr 2023, 16:46
Forum: HowTos and FAQ for TeNPy
Topic: Write MPS/MPO to disk or increase RAM?
Replies: 3
Views: 8583

Re: Write MPS/MPO to disk or increase RAM?

In terms of memory usage of DMRG, the limiting factore are usually the MPO environments rather than the MPS itself: the number of parameters for the MPS scales as O(L\chi^2 d) , where L is MPS unit cell length, chi is bond dimension and d is local dimension. In contrast, MPO environments go as O(L\c...