TeNPy Forum

Yes: just run DMRG with an initial state in the symmetry sector you're interested in.
DMRG doesn't switch charge sectors, so the charge sector of the final state is determined by your initial state.

There are more ways to get the expectation value \(E = \langle \psi | H | \psi\rangle \) of a given state than just through H_bonds: when you have a model, you can evaluate the MPO expectation value with M.H_MPO.expectation_value(psi).
Does this answer your question?

Would you mind to provide a traceback? An "incompatible LegCharge" means it's trying to contract two legs with different charges somewhere, not necessarily in the MPS bonds.

To be clear: you need to evolve this for much longer than just a few steps, and continuously take measurements for C_ij(t) . In the end, you can then fourier-transform C_ij(t) , both in space and time, to get C_k(\omega) , which is what you're really interested in, right? The maximal time you reach ...

It seems there's something gone wrong beforehand; if you try psi.test_sanity() before the psi0.apply_local_op(...) , it will likely give you an error, right? If this code is inside a loop, make sure you have actual copies psi0 = psi.copy() , not just new references. That being said: in what way can ...

Sounds like you're after the bc_shift . With bc_shift=['open', -1] for finite MPS or bc_shift=['periodic', -1] for infinite MPS, couplings in "y-direction" of the lattice wind like a helix around the cylinder. More details in this userguide section . It might also be useful to look at the ...

1) Glad that this helped. Watch out for the new version of TeNPy that will be released in the next few days! 2) You're more than welcome to contribute! There's plenty of things to be done. Hmm, I'm not completely sure, but it might just be that chi=500 is too small to reach that time. I did some sim...

If your model is conserving Sz, you should also make sure you conserve Sz during the time evolution for speedup. In this case, the expectation value splits up into \langle g.s. | \sigma_j^x(t)\sigma_0^x(0)|g.s. \rangle = \langle g.s. | \sigma_j^+(t)\sigma_0^-(0)|g.s. \rangle + \langle g.s. | \sigma_...

As far as I understand the error message, it claims that strength_vals is an integer array and hence can not divided by 2. in place . On your side, there's a very easy fix: specifying g = 1. as a float instead of g = 1 as an int - as you do in the default parameters. On TeNPy side, this is likely so...

Your couplings preserve the number of fermions and bosons independently , so you want to end up with qnumber=2 charges. Try using charges='independent' when combining the sites. Moreover, you double-count the N-terms compared to the hopping terms by explicitly adding plus_hc=True , at least if you i...

The correlation function is built with the intention that the i and j are multiple values; that's why the position r of opstr[r] is the index inside the MPS, not relative to i : opstrs = [s.Id if j % 2 == 0 else npc.expm(1.j * np.pi * Sz) for j, s in enumerate(psi.sites)] psi.correlation_function(&q...

Hi Pablo, The definition of TDVP in the context of MPS means to project the time evolution into the subspace/manifold of MPS at a fixed bond dimension. Single-Site TDVP is doing exactly that: whatever bond dimension you start with, it will keep that bond dimension. That means when you start from a p...

Short answer: Honestly, I don't know. From my limited experience with Bose-Hubbard physics, I could certainly imagine this to be a physical effect. Some suggestions for further investigation: Double-check that you're converged in the Nmax parameter of the BosonSite, i.e. how many bosons can occupy a...

I'm not completely sure if I understand what you want to do and what you mean with "Gutzwiller projection". As far as I understand, you have found an MPS of SpinHalfFermionSite -based model with both Sz and N conservation. Now you want to "project" into the subspace of fixed part...

Indeed, as far as I understand your question, you're trying to describe what the bc_shift parameter does. This one allows for a (basically) arbitrary shift when going around the cylinder; I guess you want it to be 1 or 2 in the most cases. Have you seen this section of the user-guide that I added a ...

The MPS.expectation_value method for a given \(|\phi\rangle\) gives you \(\langle\phi|A_i|\phi\rangle\), but you need
\(\langle\psi|A_i|\phi\rangle\) with \(|\phi\rangle = \exp(-i H t) B_j |\psi_0\rangle\).
The latter is exactly what MPSEnvironment.expectation_value provides.

Your new question is probably the follow up on this, so I guess I'm already a bit too late (Sorry :oops: ), but let me comment anyways. The first "standard" thing to do is to just calculate the next excited states (assuming that you have a finite system) using the "orthogonal_to"...

Wow, that sounds like some interesting physics! Sometimes, it's helpful to double-check the terms produced by the model. In your case, print(M.all_coupling_terms().to_TermList()) prints -1.00000 * B gJW_0 Bd_1 + -1.00000 * Bd gJWd_0 B_1 + -1.00000 * B gJW_1 Bd_2 + -1.00000 * Bd gJWd_1 B_2 + ... Here...

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....