TeNPy Forum

The all-up state which you start with is an *exact* eigenstate of the Heisenberg model (both the ferro-magnetic and anti-ferromagnetic one), and indeed it's the ground state (or one of them, to be precise) for the FM J=-1 case. The reason why you still go away is a bug: you're missing the Y-Y term i...

Just to make sure, you really look at the Ferromagnetic Heisenberg model, and not the transverse field Ising model, right? Why do you preserve the parity, but not Sz? To clarify: the "parity" is not the same as "spin flip symmetric". The conserved "parity" is (sum_i Z_i...

Yeah, kind of what I was expecting from what I know you've been working on...

Ugh, that's a really tough one! First of all: Is it really necessary to do the mapping from fermions to bosons, or couldn't you maybe directly implement the fermionic model instead? (I'd guess not, but just to make sure...) My first reaction definitely is "That's impossible". TeNPy's np_co...

The Ground state of the ferromagnetic Heisenberg model is degenerate, and I guess that you might get different states depending on whether you turn on parity conservation. The different ground states have different entanglement. What start did you start with? It fixes the charge sector you remain in...

As far as I know, exploiting translation symmetry or other symmetries of the lattice is not possible. There is only the trick which was pointed out in the source code of entanglement_entropy_segment before. I've implemented in 61be6397b0fedb066199bf24e948a6f6f702f6c6 today to give entanglement_entro...

Another rather minor release. We've added a list of all papers using TeNPy to the documentation. Feel free to include your own works! And a slight simplicifation, which might affect your code: using the MultiCouplingModel is no longer necessary, just use the tenpy.models.model.CouplingModel directly...

Yes, the MPS is still finite and OBC-style bc_MPS='finite' , with vectors at the boundary. It's just that the couplings in the MPO are topolically equivalent to a ring, as you drew it in your picture. Using the "default" order wouldn't change this, you would still need to define the MPS to...

You set n_max=1, i.e. you consider hard-core bosons with at most 1 particle per site, and the model has particle number conservation. The product state from which you initialize is completely filled with one particle per site, such that the hopping term can't contribute - it would create one hole an...

Take a look at the overview and readthe other "introductions". You want to use the tenpy.models.hubbard.FermiHubbardModel with the tenpy.models.lattice.Ladder (for the other ladders, implement your own "Lattice" class as described in the various introductions). The t,U,L are mode...

Version 0.7.0 and 0.7.1 have been released about 2 weeks ago and are available from pip and conda. Before updating , please read at least the section incompatible changes of the changelog. The highlight of the changes is the new support for time evolution with the W_{II} method for approximating the...

Welcome! The MPS which you pass to the TEBDEngine gets modified in place (because we want to avoid unnecessary copies of MPS in case they are large). In other words, psi_0 and eng.epsi are the same MPS (namely the time evolved one). The solution is to make a copy of the MPS at some point, e.g. psi_0...

Oops! There was a bug in init_lattice that the 'order' parameter was read out, but not used for 1D lattices (i.e. the Chain and Ladder). I've quickly fixed this in 9e020640e180d6fb9d09f534a9c91fa047cf6bab . After this fix, using order='folded' is really "enough", apart from making sure tha...

While it does not work with the version v0.6.1, it works with the latest version of the master branch. The fix is about a year old, but wasn't included into the master branch until recently when we merged the MPO evolution with the W_I/W_II approximations... I guess it's time for a version bump agai...

You're right, that's a double counting. Good catch, thanks for pointing it out! The Fermionic Hofstadter model has the same problem, of course. Additionally, one of the example models had the issue. I've fixed it in a8448f36c23fda6f2ea8217f4f8e938acb7f67df bart Did you see this? Any other consequenc...

The model is H = J \sum_{i} (0.5 (S^+_i S^-_{i+1} + h.c.) + \Delta S^z_I S^z_{i+1} ) - h^z \sum_i S^z_i , which is in the ferromagnetic phase of the XXZ chain for \Delta = -1.7 . In other words, the True ground state is simply all down states (or all up states, if you have a non-zero h^z ). Indeed, ...

Imposing a small bond dimension has slightly different effects on single-site vs. two-site DMRG. Take the extreme case of chi=1, in which case single-site DMRG (without a mixer) effectively does mean-field theory for a single site in an environment, while two-site dmrg finds the ground state of two ...

For infinite systems, the absolute value of the charges has no physical meaning but is a pure gauge choice; physical is only the difference between charge values. Indeed, the absolute value depends on the choice of your initial state - did you get different results when you initialized with the down...

So you have 3 separate conserved quantities: number of A bosons siteA, number of bosons siteB, spin SiteZ? The charges for the siteA and siteB need to be different, so you can't pass the very same Site-instance. I expect the following to work: class BosonSin(CouplingMPOModel): def init_lattice(self,...

To detect whether you have cat state, you should look at the eigenvalues of the transfermatrix, whether you get two eigenvalues exactly 1. If the ground state is a stripe, charge conservation can force it into superpositions. For that reason, I would also highly recommend to try using a larger (doub...

(I took the freedom to slightly edit your suggested code.)

May I ask why you want to create a superposition in the first place? Just for measurements? I don't really see a point for that. Or continuing some algorithm with another model which doesn't have the charge conservation? That would be more reasonable. Schollwoeck's 2011 review ("DMRG in the age...

I see. Sorry, if I appeared rude; I just wasn't sure what you wanted to do. I'd still recommend running DMRG to find the ground state, which gives you an MPS, and then just take that as initial state for the TEBD, as outlined below. On a side note, you can derive your model directly from the Nearest...

Why are you trying to do the time evolution? The paper you cited does *not* use TEBD, it uses DMRG! We define \theta = 2\pi t/T , assume adiabaticity and work in the instantaneous eigenbasis of H(\theta) . Hence, the charge transport becomes independent of the time scale T . "Adiabatically"...

No, there is no toycode.

The idea is really just to keep track of the charges, which divides the tensor into smaller blocks, and multiply only the subblocks with compatible charges, re-implementing tensordot, eigh, ... to exploit the block structure.