TeNPy Forum

To get the ground state energy within each sector, the strategy needs to be to just run DMRG for each sector separately, i.e. use charge conservation and run DMRG with an initial state in that sector. In general, turning on charge conservation should not cause any convergence issues with DMRG and es...

Indeed, psi.set_SL(...) and psi.set_SR(...) just updates the singular values, not the underlying B tensors (as you can easily see by checking the code...), so this is expected. It is meant to be a more low-level function and used by other MPS methods where you additionally also call psi.set_B(...) t...

The initial state should be chosen as product_state = ['0.0','1.0','-1.0','0.0']*(L//4) # initial state or even more simply product_state = ['0.0'] * L Note that these are strings, labeling the differen Sz=-1, 0, +1 - this is different from simple integers, which actually give you the index based on...

Just plug in "Bd Bd" and "B B", i.e. write psi.correlation_function("Bd Bd", "B B", sites1=[i], sites2=[i + 1])[0, 0] In general, the get_op function used from correlation_function and friends to translate the name into actual tensors accepts space-separated c...

It is expected that SingleSiteTDVP does not increase the bond dimension by defintion - that's how it's constructed and the reason why we have two-site TDVP in the first place (which is not strictly a TDVP anymore), and documented at the top of tdvp.py . As also stated there, the mixer is currently c...

Using bc='periodic' for the Lattice does not imply periodic boundary conditions on the MPS, but just makes the add_coupling method add the additional S_{L-1} \cdot S_0 coupling , similar as you did explicitly "by hand" in this case, such that the Hamiltonian implements the same couplings a...

why do you think you're doing something wrong? do you get error messages or "wrong" and unexpected results? If you want help, you should say that. I guess you're suprised to see all the psi.expectation_value('B') and psi.expectation_value('Bd') being zero, even in the supposedly superfluid...

Think about what exactly you're trying to calculate.... in your formula, you don't include 1.j*np.pi factors, which you seem to include in the code? I assume you want to include that. Further, you should expand \langle \exp(\sum_{i<j} 1- n_i)\rangle = \langle \prod_{i<j} \exp(1-n_i)\rangle= \langle ...

Sure, just use a corresponding 2D array for the strength should work. It's documented in add_multi_coupling , by the way, but probably not clear enough? The reference in there should have pointed to possible_multi_couplings , which directly gives you the valid shape for the strength, e.g. trying squ...

There's some issue with your models, try reading reading the model guide for more details. Some hints: You're trying to run TEBD, which requires H_bond defined in a NearestNeighbourModel. However, the PXP model is not nearest-Neighbor, so you either need to use a different time evolution method like...

Take a look at the userguide for defining new models . The idea is that you rewrite your Hamiltonian H = \sum_{\langle i,j\rangle} A_i B_j = \sum_{i} \sum_{\delta = j-i} A_i A_{i+\delta} and the nearest_neighbor_couplings for a given lattice are the set of \delta you need to sum over. To be precise,...

Hi Bart, it's essentially defined by the following line in ExactDiag.__init__() self._pipe = npc.LegPipe(legs, qconj=1, sort=(not sparse), bunch=(not sparse)) Unless you use the `sort=False` keyword, the pipe internally sorts by charge values, more precises such that it's `qmap` is lexiographically ...

The fact that you get bond dimension 2 on the boundaries is for technical reasons that we add the "IdL" (ready) and "IdR" (final) states of the MPO finite state machine on each bonds. If you don't want that, overwrite calc_H_MPO in the model by copy-pasting the current implementa...

This should really give the same. It's been a while since you wrote this - has this been resolved by now? I quickly tried to reproduce this for a random MPS, but it seemed to work as expected. If you still find differences, it would be great if you can open an issue on github and provide a (minimal ...

Try increasing/varying Lx until you see convergence! In general, if you expect symmetry breaking, the ground state with broken symmetry should be commensurate, i.e. the symmetry broken pattern needs to fit into your iDRMG unit cell! Otherwise, the MPS needs to become a "cat" state with mul...

I would recommend to use the tenpy.models.model.CouplingMPOModel , and override the methods as detailed in the model user guide . Since you have all-to-all interactions, you don't really have a chain, so you can use a TrivialLattice to write this down - but that implies that you have to manually sum...

To detect spontaneous symmetry breaking, you shouldn't check the magnetization: it is only nonzero in the symmetry broken states, but you can write an (anti)-symmetric superposition of the symmetry-broken states which has magnetization 0. Instead, you should check the decay of correlations at long d...

You have to be a bit more specific - your question is very generic, so I can only guess what your problem might be... What's your lattice, how did you get the 0, 1 and 20, 21 indices in your question? I would assume that you would rather want something like Corr=psi.term_rorrelation_function_right（[...

The python ecosystem is a bit confusing with the different ways you can have "virtual environments" in which you can install packages - see this famous xkcd comic , and the situation has not really become better since the comic came out - now you additionally have conda envrionments, and s...

I'm not sure what your Hamiltonian is, here - do you have boson hopping between the upper and lower part where you set \mu_{up} and \mu_{down} , respectively, or is there only a density-density interaction? You can only use charge conservation to preserve the respective fillings if H commutes with N...

I got the following question by email: I encountered some difficulties when setting conserved quantities. I filled two types of bosons in a two-dimensional bosonsite, and I want to control the filling numbers for each boson. Usually, I control the filling number by controlling their chemical potenti...

AFM, FM, 120°AFM mean that the system spontaneously breaks the symmetry, i.e. you have degenerate ground states. It's important to distinguish between continous and discrete symmetries, which you break here. If you enforce charge conservation, this might indeed force DMRG to write a superposition of...

1.) Hmm, good question. I never tried, but I'm afraid it wouldn't go well, and strongly advise against it. In theory, I agree that you can choose arbitary units and scale all terms in the hamiltonian accordingly. In practice, we have several points where we discard everything on the order of machine...

I guess I'm late for you to solve the problem, but maybe as a hint for people trying to solve it in the future: The #SBATCH --nodes=2 asks SLURM for 2 nodes with MPI paralellization. TeNPy can not utilize that (unless you're working in the mpi_parallel branch, which is for very specific use cases on...

TeNPy has comparable speed with other libraries like ITensor , if you fix parameters for a fair comparison. One big difference is the default dmrg_params['lanczos_params']['N_max'] , the maximum number of lanczos iterations for finding the "ground state" before moving on to the next site. ...