Expectation value of MPO tensor

[borisdevos]

Some context: I have a partition function tensor O. I find the boundary MPS fixed point psi (and environments envs) with VOMPS. I can get to the free energy through expectation_value(psi, InfiniteMPO([O]), envs).

I now have an MPO tensor E representing a different quantity (here some energy, but not a Hamiltonian term!) . I did the following:

mpo = InfiniteMPO([E]) # E is a (2,2) tensor map
expectation_value(psi, mpo, envs)

However, this gives an incorrect result. I think this is because the environments get lost in the conversion to multiline things:

MPSKit.jl/src/algorithms/expval.jl

Line 138 in 8cf041f

return prod(x -> expectation_value(x...), zip(parent(ψ), parent(mpo)))

where there is a TODO noting this.
Edit: this is also flawed because I want the expectation value of replacing 1 O tensor with E, not a chain of Es.

The best I can do right now is

MPSKit.contract_mpo_expval(psi.AC[1], envs.GLs[1], E, envs.GRs[1], psi.AC[1])

to get the right result.

Something similar to PEPSKit's contract_local_operator is probably what I want.

[lkdvos]

Yeah, using an InfiniteMPO implicitly places that everywhere so that’s definitely no good. Unfortunately this is one of those not so hard to implement but hard to come up with a nice interface problems, if you have some inspiration I’m all ears!

See also QuantumKitHub/PEPSKit.jl#117 and the discussions there