Update AdaptVQE to new, stateless VQE (#222)

* fix AdaptVQE which previously used the (now deleted) internal method _evaluate_energy of the VQE * add reno Co-authored-by: Steve Wood <[email protected]>
qiskit-community · Jun 18, 2021 · a1300f5 · a1300f5
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diff --git a/qiskit_nature/algorithms/ground_state_solvers/adapt_vqe.py b/qiskit_nature/algorithms/ground_state_solvers/adapt_vqe.py
@@ -97,12 +97,10 @@ def _compute_gradients(
             self._ansatz.operators = self._excitation_list + [exc]
             # set the current ansatz
             vqe.ansatz = self._ansatz
-            ansatz_params = vqe.ansatz._parameter_table.keys()
-            # construct the expectation operator of the VQE
-            vqe._expect_op = vqe.construct_expectation(ansatz_params, self._main_operator)
             # evaluate energies
             parameter_sets = theta + [-self._delta] + theta + [self._delta]
-            energy_results = vqe._energy_evaluation(np.asarray(parameter_sets))
+            energy_evaluation = vqe.get_energy_evaluation(self._main_operator)
+            energy_results = energy_evaluation(np.asarray(parameter_sets))
             # compute gradient
             gradient = (energy_results[0] - energy_results[1]) / (2 * self._delta)
             res.append((np.abs(gradient), exc))

diff --git a/releasenotes/notes/fix-adapt-vqe-c9df0c06ea352b91.yaml b/releasenotes/notes/fix-adapt-vqe-c9df0c06ea352b91.yaml
@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fix :class:`~qiskit_nature.algorithms.AdaptVQE` after the update of 
+    :class:`~qiskit.algorithms.VQE` which deleted the internal ``_energy_evaluation``
+    method that Adapt-VQE was relying on.