Fermionic OpSum to TTN constructor

[b-kloss]

This PR generalizes the construction of TTNs from OpSums to systems with fermionic degrees of freedom when ITensors.using_auto_fermion()==true.
The implementation closely follows the logic in the OpSum to MPO constructor in ITensors.jl (in fact replicates quite a bit of code) and does not expose the auto_fermion logic.
The functionality passes tests (test/test_opsum_to_ttno.jl) for fermions and electrons.

Tests added:

• test_opsum_ttn.jl: tests for non-fermionic QN inds on comb graph, and comb graph with a single internal vertex. These tests contract the Hamiltonian TTNO and compare with one generated by ITensors.MPO.
• test_opsum_ttn.jl: tests for fermionic QN inds on comb graph for NNN tight-binding model on comb graph. Since comparison \approxeq between fermionic ITensors is broken, the Hamiltonian is cast into a dense matrix for both TTN and MPO format (using Combiners) and compared.
• test_treetensornetworks/test_solvers/test_dmrg.jl: tests added for NNN heisenberg and hubbard model on comb graph, comparison of ground state energies with result from ITensors.dmrg. Tests for single-site DMRG with QNs have been removed due to missing random_ttn and lack of subspace_expansion.

To Do:

• Implement fermionic permutation sign in ttn_svd.
• Cleanup convoluted (fermion/arrow-direction related) logic inside svd_ttn.
• Implement tests for fermionic systems.
• Test DMRG for NNN Hubbard Model on comb tree.
• Figure out whether comparison of fermionic ITensors is broken.
• Add more tests for solvers including fermionic systems?
• Add test that verifies that vanishing coefficient terms do not influence final bond dimension. (Probably better in another PR since it's unrelated to the fermion implementation.)

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (5ec5228) 72.55% compared to head (5a24c2a) 72.90%.

Files	Patch %	Lines
src/models.jl	98.27%	1 Missing ⚠️

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@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
+ Coverage   72.55%   72.90%   +0.35%     
==========================================
  Files          71       71              
  Lines        4055     4097      +42     
==========================================
+ Hits         2942     2987      +45     
+ Misses       1113     1110       -3     

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[b-kloss]

@mtfishman @emstoudenmire
It looks like svd_ttn is working for fermions now and the logic for the fermion-sign really only comes in for the permutation w.r.t. the direct product in the last step.

[emstoudenmire]

Nice! So did you find & fix that other bug that was showing up for electrons but was related to QNs?

[b-kloss]

That one turned out to be a typo in the Hubbard model OpSum that made it have nonzero flux.

…

On Fri, Jan 12, 2024, 7:06 PM Miles ***@***.***> wrote: Nice! So did you find & fix that other bug that was showing up for electrons but was related to QNs? — Reply to this email directly, view it on GitHub <#122 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AGL4SEJ3LNVSZBG6GVKEVQTYOHFW3AVCNFSM6AAAAABBXQN3V6VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTQOJQGE3DGNJQGQ> . You are receiving this because you authored the thread.Message ID: ***@***.***>

[emstoudenmire]

I'm excited to hear it's already working. Is it passing tests on multiple fermionic cases? If so this is a big development.

[b-kloss]

It's working for spinless fermions and the Hubbard model (including next-nearest neighbors in both cases).
DMRG results on a tree agree with MPS calculations.
Comparing two fermionic ITensors @test A ≈ B seems to be broken but I checked by casting to a Dense matrix that the elements agree.

[emstoudenmire]

Those are some pretty strenuous tests. Great. We can discuss the failing ITensor comparison test, since maybe that's more of a low-level bug with ITensors.jl ?

[mtfishman]

Looks good, thanks a lot @b-kloss. This is a big step forward for ITensor!

I think it is ready to merge once the new round of comments are addressed.

[mtfishman]

Looks good, thanks again. I'll merge once tests pass.