Fix existing initial points with reps (#808) (#812)

* Repeat initial point parameters based on ansatz reps.

* Add release note.

* Add tests.

* Account for reps in initial_points tests.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit 88bf668)

Co-authored-by: Declan Millar <declan.millar@ibm.com>

qiskit_nature/algorithms/initial_points/hf_initial_point.py

@@ -147,7 +147,8 @@ def compute(
         self._compute()
 
     def _compute(self) -> None:
-        self._parameters = np.zeros(len(self._excitation_list))
+        reps = self._ansatz.reps if self._ansatz is not None else 1
+        self._parameters = np.zeros(reps * len(self._excitation_list))
 
     def get_energy(self) -> float:
         """The reference energy.

qiskit_nature/algorithms/initial_points/mp2_initial_point.py

@@ -312,7 +312,9 @@ def to_numpy_array(self) -> np.ndarray:
         """The initial point as an array."""
         if self._corrections is None:
             self.compute()
-        return np.asarray([correction.coefficient for correction in self._corrections])
+        coefficients = np.asarray([correction.coefficient for correction in self._corrections])
+        reps = self._ansatz.reps if self._ansatz is not None else 1
+        return np.tile(coefficients, reps)
 
     def get_energy_corrections(self) -> np.ndarray:
         """The energy corrections for each excitation."""

qiskit_nature/algorithms/initial_points/vscf_initial_point.py

@@ -129,5 +129,5 @@ def compute(
                 "Set the ansatz or call compute with it as an argument. "
                 "The ansatz is not required if the excitation list has been set directly."
             )
-
-        self._parameters = np.zeros(len(self._excitation_list))
+        reps = self._ansatz.reps if self._ansatz is not None else 1
+        self._parameters = np.zeros(reps * len(self._excitation_list))

releasenotes/notes/fix-existing-initial-point-with-reps-5ca0fd2defe00539.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fix the initial point classes to account for the number of times the evolved operators are
+    repeated in the ansatz.

test/algorithms/ground_state_solvers/test_groundstate_eigensolver.py

@@ -64,6 +64,7 @@ def setUp(self):
         algorithm_globals.random_seed = self.seed
 
         self.reference_energy = -1.1373060356951838
+        self.mp2_initial_point = [0.0, 0.0, -0.07197145]
 
         self.qubit_converter = QubitConverter(JordanWignerMapper())
         self.electronic_structure_problem = ElectronicStructureProblem(self.driver)
@@ -583,7 +584,55 @@ def test_vqe_ucc_factory_with_mp2(self):
         res = calc.solve(self.electronic_structure_problem)
 
         np.testing.assert_array_almost_equal(
-            solver.initial_point.to_numpy_array(), [0.0, 0.0, -0.07197145]
+            solver.initial_point.to_numpy_array(), self.mp2_initial_point
+        )
+        self.assertAlmostEqual(res.total_energies[0], self.reference_energy, places=6)
+
+    def test_vqe_ucc_factory_with_reps(self):
+        """Test when using the default initial point with repeated evolved operators."""
+        ansatz = UCCSD(
+            qubit_converter=self.qubit_converter,
+            num_particles=self.num_particles,
+            num_spin_orbitals=self.num_spin_orbitals,
+            reps=2,
+        )
+
+        solver = VQEUCCFactory(
+            ansatz=ansatz,
+            quantum_instance=QuantumInstance(BasicAer.get_backend("statevector_simulator")),
+        )
+        calc = GroundStateEigensolver(self.qubit_converter, solver)
+        res = calc.solve(self.electronic_structure_problem)
+
+        np.testing.assert_array_almost_equal(
+            solver.initial_point.to_numpy_array(), np.zeros(6, dtype=float)
+        )
+        self.assertAlmostEqual(res.total_energies[0], self.reference_energy, places=6)
+
+    def test_vqe_ucc_factory_with_mp2_with_reps(self):
+        """Test when using MP2InitialPoint to generate the initial point with repeated evolved
+        operators.
+        """
+
+        initial_point = MP2InitialPoint()
+
+        ansatz = UCCSD(
+            qubit_converter=self.qubit_converter,
+            num_particles=self.num_particles,
+            num_spin_orbitals=self.num_spin_orbitals,
+            reps=2,
+        )
+
+        solver = VQEUCCFactory(
+            ansatz=ansatz,
+            quantum_instance=QuantumInstance(BasicAer.get_backend("statevector_simulator")),
+            initial_point=initial_point,
+        )
+        calc = GroundStateEigensolver(self.qubit_converter, solver)
+        res = calc.solve(self.electronic_structure_problem)
+
+        np.testing.assert_array_almost_equal(
+            solver.initial_point.to_numpy_array(), np.tile(self.mp2_initial_point, 2)
         )
         self.assertAlmostEqual(res.total_energies[0], self.reference_energy, places=6)
 

test/algorithms/initial_points/test_hf_initial_point.py

@@ -46,6 +46,7 @@ def test_missing_ansatz(self):
     def test_set_get_ansatz(self):
         """Test set get ansatz."""
         ansatz = Mock(spec=UCC)
+        ansatz.reps = 1
         ansatz.excitation_list = self.excitation_list
         self.hf_initial_point.ansatz = ansatz
         self.assertEqual(ansatz, self.hf_initial_point.ansatz)
@@ -89,6 +90,7 @@ def test_compute(self):
         """Test length of HF initial point array."""
         grouped_property = Mock(spec=GroupedSecondQuantizedProperty)
         ansatz = Mock(spec=UCC)
+        ansatz.reps = 1
         ansatz.excitation_list = self.excitation_list
         self.hf_initial_point.compute(ansatz=ansatz, grouped_property=grouped_property)
         initial_point = self.hf_initial_point.to_numpy_array()

test/algorithms/initial_points/test_mp2_initial_point.py

@@ -55,6 +55,7 @@ def setUp(self):
 
         self.excitation_list = [[[0], [1]]]
         self.mock_ansatz = Mock(spec=UCC)
+        self.mock_ansatz.reps = 1
         self.mock_ansatz.excitation_list = self.excitation_list
 
         electronic_energy = Mock(spec=ElectronicEnergy)
@@ -133,6 +134,7 @@ def test_no_orbital_energies(self):
         self.mock_grouped_property.get_property = Mock(return_value=electronic_energy)
 
         ansatz = Mock(spec=UCC)
+        ansatz.reps = 1
         ansatz.excitation_list = self.excitation_list
 
         mp2_initial_point = MP2InitialPoint()
@@ -239,6 +241,7 @@ def test_mp2_initial_point_with_real_molecules(
         driver_result = problem.grouped_property_transformed
 
         ansatz = Mock(spec=UCC)
+        ansatz.reps = 1
         ansatz.excitation_list = excitations
 
         mp2_initial_point = MP2InitialPoint()

test/algorithms/initial_points/test_vscf_initial_point.py

@@ -43,6 +43,7 @@ def test_missing_ansatz(self):
     def test_set_get_ansatz(self):
         """Test set get ansatz."""
         ansatz = Mock(spec=UVCC)
+        ansatz.reps = 1
         ansatz.excitation_list = self.excitation_list
         self.vscf_initial_point.ansatz = ansatz
         self.assertEqual(ansatz, self.vscf_initial_point.ansatz)
@@ -64,6 +65,7 @@ def test_vscf_with_excitation_list_set_directly(self):
     def test_vscf_compute(self):
         """Test VSCF initial point is all zero when called via compute."""
         ansatz = Mock(spec=UVCC)
+        ansatz.reps = 1
         ansatz.excitation_list = self.excitation_list
         self.vscf_initial_point.compute(ansatz)
         initial_point = self.vscf_initial_point.to_numpy_array()