TeNPy Forum

The MPS does not know about the Hamiltonian, so it can't have an "energy" per se. Instead, you have a model you ran DMRG with, and you can compute the energy from the MPO of that model as E = model.H_MPO.expectation_value(psi) giving you the value E = \langle \psi | H |\psi \rangle . (For ...

Hi Tana, first of all, welcome to TeNPy, and sorry that I didn't reply earlier. I hope this still helps. The issue which you ran into is quite simple: the SpinHalfFermionSite represents a 4-dimensional local Hilbert space (with states "empty", "spin-up", "spin-down", &q...

Hmmm. I'm still a bit worried, because after all we should usually avoid taking inverses of small singular values all-together for numerical stability. I tried to track down where it came from and found that it basically stems from this line in MPS.compress_svd() not truncating tiny singualar values...

Hi Ruben, I have seen this warning before as well, but not so much lately. I vaguely remember that I also tried printing the S values and/or checking that none of them was 0 or negative, but couldn't quite pinpoint it.When testing S=S**-1. for different values like S = np.array([1.e-2, 1.e-15, 1.e-2...

I played around with this a little bit. When you choose a smaller time step, the error gets smaller, it's O(dt). Further, I noticed that the error is coming from the state not being in canonical form: If you call psi.canonical_form_finite() right before measurements, at least this error is gone, as ...

Why do you need the state Sx|psi> in the first place? As you know, S^x=0.5* (S^+ + S^-) and thus applying Sx to a state S|\psi\rangle with fixed Sz will give you a superposition of |\phi^\pm\rangle := S^\pm |\psi\rangle . In other words, S^x |\psi\rangle does not have a well defined Sz charge, and c...

Sorry for the late reply. In general, I think DMRG should be suitable for such problems. I could understand that it fails for tiny/zero values of lambda, but for O(1) values of lambda it should definitely work. (In fact, I think that people have successfully used DMRG as an impurity solver for DMFT....

Your code looks right. I would have done it basically the same way :) In the newest version of TeNPy, tenpy.networks.site.multi_sites_combine_charges is deprecated in favor of tenpy.networks.site.set_common_charges , you should call it with set_common_charges([siteA, siteB], 'independent') . This sh...

the line states = dmrg_params['orthogonal_to'] = [] creates a new list and assigns it to both the variable name states and dmrg_params['orthogonal_to'] . Remember, in Python everything is an "object", and your variables really are just references to objects. Inside the loop, you first crea...

1) How did you try to install TeNPy? It looks like it's using a version installed with `pip` into the local home directory. Can you try to pip uninstall physics-tenpy and then try to re-install TeNPy? Moreover, the svd_gesvd hints that you're using a very old version of scipy? What python are you us...

1) That was intentional, you need a new copy each time in the loop! After all, you want to have a list of 3 different states in the end. You might at most argue that it's a bad idea to "restart" the whole DMRG from scratch for the excited states, if we already found a ground state. But the...

This wasn't implemented yet in TeNPy , as you have probably seen by the ToDo entry. However, since today, it is: 64cd7a3a690e80b2876fce48e79675f060b52ab7 ;) The function from_infiniteT_canonical gives you an initial density matrix with fixed particle number on the physical sector, i.e. the canonical...

Welcome! This forum is meant to be a community place for both the users and developers of the Tensor Network Python (TeNPy) package. The source code of this package is available on github . The official documentation (generated from the latest source code) has also a user-guide section with some tu...

Even though the model is originially nearest-neighbor on the periodic chain, it will have longer-range interactions in the (open-boundary) MPS - with default ordering, there is one interaction going from site 0 to site L-1, or if you use the "folded" order, most of the originally nearest-n...

That's why the warning is there - to help you spot typos

Well, TEBD is nice because it is conceptually very simple - you just use a Suzuki-Trotter decomposition and straight forward iterations of (apply two site gate; compress & split to get back to MPS form). Grouping sites makes things more costly, however - formally TEBD scales as \mathcal{O}(\chi^...

First of all, let me clarify: Do you want some uniform "background density" and then some more particles in the center? Or do you only want particles in the center? For 1): If you don't enforce charge conservation, DMRG is "allowed" to switch to another charge sector, even if you...

you need to make a loop and run DMRG multiple times, each time orthogonalizing against the previous state: psi0 = MPS.from_product_state(...) states = dmrg_params['orthogonal_to'] = [] for i in range(3): psi = psi0.copy() results = dmrg.run(psi, M, dmrg_params) states.append(psi) # this also adds th...

Let's look at the example Ladder image: https://tenpy.readthedocs.io/en/latest/reference/tenpy-models-lattice-Ladder-1.png The MPS follows the red dashed line 0 - 1 - 2 -3 -4 -5- ... When you cut the MPS at the bond between 3 and 4, it will split the system in regions left = {0, 1, 2, 3} and right={...

If you have at most next-nearest neighbor interactions in a 1D chain, you can group sites and then perform TEBD. In general, I'd recommend to switch to another time evolution algorithm, though: either TDVP or the tenpy.algorithms.mpo_evolution.ExpMPOEvolution can deal with longer range. The latter e...

Welcome, Rike!

chi_min can only avoid reducing the bond dimension, not increase it further.
In addition to setting chi_min to the same value as chi_max, you should set svd_min to 0.

Clearly, you can only do this for small system sizes, where you can also do a full exact diagonalization. DMRG really is not a good algorithm at finding excited states, as I explained in the earlier posting You should just use exact diagonalization to find (all) the excited states, and then convert ...

I'm not sure what exactly you mean with "occupation-number-representation basis". You have a local basis of occupations, e.g. for spin-less fermions just |0\rangle, |1\rangle> = c^\dagger |0\rangle , or for spin-full fermions |0\rangle , |\uparrow\rangle \equiv c^\dagger_\uparrow|0\rangle,...