Search found 132 matches

by Johannes
01 Apr 2020, 23:18
Forum: HowTos and FAQ for TeNPy
Topic: Excited states of effective Hamiltonian
Replies: 1
Views: 27

Re: Excited states of effective Hamiltonian

Indeed, the guess should be very close to the actual ground state if DMRG converged well; this will lead to problems for Lanczos. Did choosing a different initial state help? You can also try to e.g. apply Sx or Sz operators on the initial guess of the ground state, if you have some idea of what the...
by Johannes
23 Mar 2020, 23:40
Forum: HowTos and FAQ for TeNPy
Topic: Adding disorders and adding a site to 1D SSH chain
Replies: 2
Views: 333

Re: Adding disorders and adding a site to 1D SSH chain

You want to simulate it on a finite system, right? The SSH chain is just a regular chain with alternating coupling strength. Given that, I think it might be easier in this case to really use the "Chain" with a single-site unit cell instead of a Lattice with a two-site unit cell. In particular, (4) i...
by Johannes
23 Mar 2020, 18:41
Forum: HowTos and FAQ for TeNPy
Topic: 2D DMRG with boundary conditions and lattice index
Replies: 2
Views: 46

Re: 2D DMRG with boundary conditions and lattice index

(1): Looks good to me. In fact, that's code taken from CouplingMPOModel.init_lattice() for the case of a 2D lattice. (2): Yes. You might want to take a look at mps2lat_values in that case. Use it like this: exp_vals_mps = psi.expectation_value("N") exp_vals_lat = model.lat.mps2lat_values(exp_vals_mp...
by Johannes
11 Mar 2020, 16:36
Forum: Algorithms
Topic: Kinetic energy
Replies: 2
Views: 730

Re: Kinetic energy

I think the question aims at how to get the terms contained in the kinetic energy. Actually, there's some functionality in the tenpy.models.model.CouplingModel , which is not explicitly used in most of the pre-defined models, though, and a bit hidden in the documentation. When one calls add_coupling...
by Johannes
11 Mar 2020, 05:12
Forum: HowTos and FAQ for TeNPy
Topic: leading eigenvector of transfer matrix
Replies: 6
Views: 628

Re: leading eigenvector of transfer matrix

You cannot/should not take the transfer matrix of a single site, if you have a two-site unit cell in your MPS. Sometimes, the charge values will not even allow you, and even if they do, it there is a different gauge on the two bonds - which can and will be the case if there are degenerate singular v...
by Johannes
11 Mar 2020, 04:10
Forum: HowTos and FAQ for TeNPy
Topic: leading eigenvector of transfer matrix
Replies: 6
Views: 628

Re: leading eigenvector of transfer matrix

BTW, in this sense, the normalization factors of the left (to square singular value) and right eigenvector (identity) of right canonical form should cancel each other out, right? However, in a real case it seems will change the largest amplitude of the transfer matrix (the largest eigenvalue will b...
by Johannes
10 Mar 2020, 23:23
Forum: HowTos and FAQ for TeNPy
Topic: leading eigenvector of transfer matrix
Replies: 6
Views: 628

Re: leading eigenvector of transfer matrix

It's just a matter of normalization.
Eigenvalues get normalized to np.linalg.norm(v) = 1, but in this case you want np.sum(v) = 1.

At the end of your code, add

Code: Select all

vdiag = v.diagonal()
print(vdiag/np.sum(vdiag))
print(psi.get_SL(0)**2)
and compare the results, the prints should give the same
by Johannes
05 Mar 2020, 00:02
Forum: HowTos and FAQ for TeNPy
Topic: Semi-infinite Ising chain with boundary
Replies: 1
Views: 858

Re: Semi-infinite Ising chain with boundary

That's an excellent question! TeNPy supports 'segment' boundary conditions, which you can basically use to do that. What we had in mind for those boundary conditions is the folling: Run DMRG in an infinite system Extract or "cut out" a subsystem with a finite length from it, giving you an MPS with '...
by Johannes
07 Feb 2020, 08:18
Forum: Implementations
Topic: Readthedocs vs Github pages
Replies: 4
Views: 1041

Re: Readthedocs vs Github pages

Okay, I paid for the Gold membership of readthedocs to remove the ads (That's the drawback, it costs money. But I don't mind supporting ReadTheDocs with a few dollars a month, their service to the community is great :) ) I've also updated the references from within the TeNPy repository and the links...
by Johannes
07 Feb 2020, 06:14
Forum: HowTos and FAQ for TeNPy
Topic: Problems with modelling a fermionic ladder
Replies: 3
Views: 334

Re: Problems with modelling a fermionic ladder

Now I face the problem that in order to see the Ising-like phase transition I want to calculate the correlation function <\Psi_0 |T^y_i T^y_{i+r} |\Psi_0> with T^y_i= \frac{1}{2}(-i c^\dagger_{i,u}c_{i,d}+i c^\dagger_{i,d}c_{i,u}) where u and d are again the rungs of the ladder. Since the Ladder ge...