Fix #192 (#193) (#195)

* Add regression test against #192 * Fix #192 * Add reno * Run black Co-authored-by: Max Rossmannek <[email protected]>
qiskit-community · May 19, 2021 · 00b6091 · 00b6091
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Showing 3 changed files with 33 additions and 6 deletions.
diff --git a/qiskit_nature/problems/second_quantization/electronic/electronic_structure_problem.py b/qiskit_nature/problems/second_quantization/electronic/electronic_structure_problem.py
@@ -148,11 +148,10 @@ def symmetry_sector_locator(self, z2_symmetries: Z2Symmetries) -> Optional[List[
         Returns:
             The sector of the tapered operators with the problem solution.
         """
-        q_molecule = cast(QMolecule, self._molecule_data_transformed)
-
         hf_bitstr = hartree_fock_bitstring(
-            num_spin_orbitals=2 * q_molecule.num_molecular_orbitals,
-            num_particles=self.num_particles)
+            num_spin_orbitals=z2_symmetries.symmetries[0].num_qubits,
+            num_particles=self.num_particles,
+        )
         sector_locator = ElectronicStructureProblem._pick_sector(z2_symmetries, hf_bitstr)
 
         return sector_locator

diff --git a/releasenotes/notes/fix-incompatibility-of-freeze-core-and-z2symmetry-1b5caa71986abaee.yaml b/releasenotes/notes/fix-incompatibility-of-freeze-core-and-z2symmetry-1b5caa71986abaee.yaml
@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    The `FreezeCoreTransformer` (and `ActiveSpaceTransformer`) were incompatible with the automatic
+    `Z2Symmetry` reduction. This issue was fixed by correcting the
+    `ElectronicStructureProblem.symmetry_sector_locator` method.
diff --git a/test/algorithms/ground_state_solvers/test_groundstate_eigensolver.py b/test/algorithms/ground_state_solvers/test_groundstate_eigensolver.py
@@ -33,8 +33,10 @@
 from qiskit_nature.mappers.second_quantization import JordanWignerMapper, ParityMapper
 from qiskit_nature.converters.second_quantization import QubitConverter
 from qiskit_nature.problems.second_quantization import ElectronicStructureProblem
-from qiskit_nature.problems.second_quantization.electronic.builders.fermionic_op_builder import \
-    build_ferm_op_from_ints
+from qiskit_nature.problems.second_quantization.electronic.builders.fermionic_op_builder import (
+    build_ferm_op_from_ints,
+)
+from qiskit_nature.transformers import FreezeCoreTransformer
 
 
 class TestGroundStateEigensolver(QiskitNatureTestCase):
@@ -376,6 +378,26 @@ def test_uccsd_hf_aer_qasm_snapshot(self):
         result = gsc.solve(self.electronic_structure_problem)
         self.assertAlmostEqual(result.total_energies[0], self.reference_energy, places=3)
 
+    def test_freeze_core_z2_symmetry_compatibility(self):
+        """Regression test against #192.
+
+        An issue arose when the FreezeCoreTransformer was combined with the automatic Z2Symmetry
+        reduction. This regression test ensures that this behavior remains fixed.
+        """
+        driver = HDF5Driver(hdf5_input=self.get_resource_path("LiH_sto3g.hdf5", "transformers"))
+        problem = ElectronicStructureProblem(driver, [FreezeCoreTransformer()])
+        qubit_converter = QubitConverter(
+            ParityMapper(),
+            two_qubit_reduction=True,
+            z2symmetry_reduction="auto",
+        )
+
+        solver = NumPyMinimumEigensolverFactory()
+        gsc = GroundStateEigensolver(qubit_converter, solver)
+
+        result = gsc.solve(problem)
+        self.assertAlmostEqual(result.total_energies[0], -7.882, places=2)
+
 
 if __name__ == '__main__':
     unittest.main()