Not expected ordering of parameters in energy evaluation call

[dgiannelli]

Information

• Qiskit Aqua version: 0.7., 0.8. up to latest 0.8.1

What is the current behavior?

Using VQE class with a parametrized quantum circuit (i.e. efficientsu2), the energy evaluation function is called with parameters in alphabetical order, and not in numerical order as it happens with method assign_parameters of parametrized quantum circuit.

This leads, for example, to parameters called in order: theta[0], theta[10], theta[11], theta[1], etc... which can cause unexpected behaviors if a program relies on numerical ordering of parameters.

What is the expected behavior?

That the parameters are called in numerical ordering: theta[0], theta[1], theta[2], etc...

Suggested solutions

For the case of parametrized quantum circuit, the class VQAlgorithm in "qiskit-aqua/qiskit/aqua/algorithms/vq_algorithm.py" sorts the parameters in the following way:

self._var_form_params = sorted(var_form.parameters, key=lambda p: p.name)

while it could behave like the parametrized circuit method assign_parameters, which calls them according to the order of the property ordered_parameters.

[woodsp-ibm]

See #5614

[nonhermitian]

Or see #5557.

[Cryoris]

This is a work in progress and should be fixed soon! 🙂

[Cryoris]

This should be fixed by #6418 (see the sort_parameters_by_name argument), so I'll go ahead and close this issue 🙂