[ENHANCEMENT] OpSum term to ITensor

[nicolasloizeau]

Is your feature request related to a problem? Please describe.

It would be nice to be able to easily convert an OpSum term to a ITensor. This would allow to easily make a list of gates for TEBD out of an OpSum.

Describe the solution you'd like
I want to be able to do something like :

op(O[i])

where O is an OpSum.

[mtfishman]

Agreed, this would be nice to have. I've partially implemented this in various places but not officially in ITensorMPS.jl yet. I think the hard part about making this an official feature is handling all of the various corner cases, such as fermions. For the time being, you can always first construct an MPO and then contract it with prod to create an ITensor.

We have ongoing work rewriting ITensors.jl and the OpSum system, and that will include more systematic support for an automatic fermion sign system, so I think it will be easier to implement this as part of that rewrite.