Qiskit VQE output

Question

When running Qiskit vqe.compute_minumum_eigenvalue() a dictionary of various values are returned.

Within this dictionary are two parameters called cost_function_evals and optimizer_evals. The first always contains some number whereas optimizer_evals = None.

My question is, are these two the same?

In the source code I see no place where the value optimizer_evals is actually updated, but cost_function_evals is. Here is the source code: https://qiskit.org/documentation/_modules/qiskit/algorithms/minimum_eigen_solvers/vqe.html

Answer 1

I think you are right that these two attributes are supposed to the same. Looking at the git history, optimizer_evals was used prior to this PR: https://github.com/Qiskit/qiskit-terra/pull/6418 but the usage was removed during the refactoring in that PR.

Prior to #6418, cost_function_evals was set by _eval_count which in turn was set by optimizer_evals.

L460 self._ret.cost_function_evals = self._eval_count

L443 if vqresult.optimizer_evals is not None and self._eval_count >= vqresult.optimizer_evals:
L444    self._eval_count = vqresult.optimizer_evals

in https://github.com/Qiskit/qiskit-terra/blob/5ec42789171b001780239d9e03e5dba4e077aa40/qiskit/algorithms/minimum_eigen_solvers/vqe.py

opt_params, opt_val, num_optimizer_evals = optimizer.optimize(
    nparms,
    cost_fn,
    variable_bounds=bounds,
    initial_point=initial_point,
    gradient_function=gradient_fn,
)
eval_time = time.time() - start


result = VariationalResult()
result.optimizer_evals = num_optimizer_evals

in https://github.com/Qiskit/qiskit-terra/blob/5ec42789171b001780239d9e03e5dba4e077aa40/qiskit/algorithms/variational_algorithm.py#L229-L239

After #6418, cost_functions_evals are directly set by optimizer.nfev and therefore the intermediate optimizer_evals are no longer used.

opt_result = self.optimizer.minimize(
    fun=energy_evaluation, x0=initial_point, jac=gradient, bounds=bounds
)


eval_time = time() - start_time


result = VQEResult()
result.optimal_point = opt_result.x
result.optimal_parameters = dict(zip(self.ansatz.parameters, opt_result.x))
result.optimal_value = opt_result.fun
result.cost_function_evals = opt_result.nfev

https://github.com/Qiskit/qiskit-terra/blob/dd7f9390cf076f9545fa6086de486f2c06764ea5/qiskit/algorithms/minimum_eigen_solvers/vqe.py#L546-L556