MPO tensors

[Jakob-Unfried]

Different strategy:

• In sites, cache a basis for four-leg tensors, with labels
• use this basis when decomposing couplings into mini MPOs
• can use these labels in the MPO graph
• Maybe keep the current idea anyway, for later. It basically encodes the IdL and IdR ideas.
• Name?
  • mpo_building_block is bad
  • maybe CBD (coupling building block)

[LudwigZw]

Changes for tenpy MPOs:

• MPOGraph attribute _coupling_tensors with hash: tensor dict
• change .graph to tuples containing hash

Changes for tenpy Couplingmodel:

• change structure to add generic coupling (cyten class Coupling), use list of int for sites

For couplings:

• Method to insert identity tensors on virtual legs (with fixed braiding convention)

[Jakob-Unfried]

See tenpy/tenpy#590
and tenpy/tenpy#611

[jhauschild]

Thanks for pointing that out, Jakob. I think we can first do the minimal set of changes to get things to work with the current model structure, and then consider tenpy/tenpy#590 and tenpy/tenpy#611 as separate issue.