Default bounds for UCC ansatz circuits

What is the expected enhancement?

Bounds are required for many optimizers (all of the package scikit-quant as can be read on page 9 of their documentation and possibly some of the other optimizers).

Most of the ansatzes located in qiskit terra (EfficientSU2, ExcitationPreservation, …) have default bounds for the parameters (-pi to pi). See e.g. method parameter_bounds of EfficientSU2.

The ansatzes in qiskit nature (UCC, …) don’t have these default bounds implemented. Thus, those VQE calculations fail as pointed out in this bug report.

So, I propose to add the parameter_bounds method also to the ansatzes is qiskit nature.

I’ll gladly help adding the code.

[mrossinek]

I will raise this idea to the people for involved with the affected components of Qiskit Terra and get back to you 👍

[cpossel]

The question now is whether float("inf") should be the default instead of None (similarly to EfficientSU2 just hard-coded in UCC) or to dynamically set it to inf if the optimizer requires it. This would be possible within vqe.py or rather in the function validate_bounds called by VQE (located in validate_bounds.py). There the VQE can actually check whether the optimizer needs bounds and then could set them if required. This would have the advantage that these arbitrary bounds would be inserted only when needed (allowing for unbounded optimizers to work properly).

As soon as this question is clarified, I’ll gladly add some code to implement it and open a PR (though in the latter case it would be in qiskit terra code).