MPO construction from two site custom op

[ryanlevy]

See Miles’ answer from here:

Defining two site local operators as part of a local Hamiltonian

It’s an interesting thing to do, and would be a good feature for us to have, but unfortunately right now the AutoMPO system (since renamed the OpSum system) doesn’t support two-site operators like “CZ” currently. Internally what it would have to do is to compute the SVD of this operator to write it as a sum of products of single-site operators, and then put these into the sum of terms (like a rewriting rule). So it’s something we could add but would take a bit of development. As a workaround, y…

Essentially the OpSum system can’t do that at the moment, however if you can write it as a string (O_i \otimes O_j \dots ) or a sum of strings as in the post then that can be used. And for generic 2 site matrices you would have to decompose it by hand into that format