Chemistry auto Z2 symmetry reduction support (qiskit-community/qiskit-aqua#870)

* Automatic Z2Symmetry reduction support

* Automatic Z2Symmetry reduction support

Author: woodsp-ibm

qiskit/aqua/algorithms/eigen_solvers/numpy_eigen_solver.py

@@ -103,7 +103,8 @@ def aux_operators(self, aux_operators: List[BaseOperator]) -> None:
             aux_operators = \
                 [aux_operators] if not isinstance(aux_operators, list) else aux_operators
             self._aux_operators = \
-                [op_converter.to_matrix_operator(aux_op) for aux_op in aux_operators]
+                [op_converter.to_matrix_operator(aux_op) if aux_op is not None else None
+                 for aux_op in aux_operators]
 
     @property
     def k(self) -> int:
@@ -164,14 +165,17 @@ def _get_energies(self):
             energies[i] = self._ret['eigvals'][i].real
         self._ret['energies'] = energies
         if self._aux_operators:
-            aux_op_vals = np.empty([self._k, len(self._aux_operators), 2])
+            aux_op_vals = []
             for i in range(self._k):
-                aux_op_vals[i, :] = self._eval_aux_operators(self._ret['eigvecs'][i])
+                aux_op_vals.append(self._eval_aux_operators(self._ret['eigvecs'][i]))
             self._ret['aux_ops'] = aux_op_vals
 
     def _eval_aux_operators(self, wavefn, threshold=1e-12):
         values = []
         for operator in self._aux_operators:
+            if operator is None:
+                values.append(None)
+                continue
             value = 0.0
             if not operator.is_empty():
                 value, _ = operator.evaluate_with_statevector(wavefn)

qiskit/aqua/algorithms/minimum_eigen_solvers/vqe.py

@@ -328,7 +328,7 @@ def _eval_aux_ops(self, threshold=1e-12, params=None):
         values = []
         params = []
         for idx, operator in enumerate(self._aux_operators):
-            if not operator.is_empty():
+            if operator is not None and not operator.is_empty():
                 temp_circuit = QuantumCircuit() + wavefn_circuit
                 circuit = operator.construct_evaluation_circuit(
                     wave_function=temp_circuit,
@@ -345,6 +345,9 @@ def _eval_aux_ops(self, threshold=1e-12, params=None):
             result = self._quantum_instance.execute(to_be_simulated_circuits)
 
             for idx, operator in enumerate(self._aux_operators):
+                if operator is None:
+                    values.append(None)
+                    continue
                 if operator.is_empty():
                     mean, std = 0.0, 0.0
                 else:
@@ -358,8 +361,7 @@ def _eval_aux_ops(self, threshold=1e-12, params=None):
                 values.append((mean, std))
 
         if values:
-            aux_op_vals = np.empty([1, len(self._aux_operators), 2])
-            aux_op_vals[0, :] = np.asarray(values)
+            aux_op_vals = [np.asarray(values)]
             self._ret['aux_ops'] = aux_op_vals
 
     def compute_minimum_eigenvalue(
@@ -390,6 +392,8 @@ def _run(self) -> 'VQEResult':
             self._operator = \
                 self._config_the_best_mode(self._operator, self._quantum_instance.backend)
             for i in range(len(self._aux_operators)):
+                if self._aux_operators[i] is None:
+                    continue
                 if not self._aux_operators[i].is_empty():
                     self._aux_operators[i] = \
                         self._config_the_best_mode(self._aux_operators[i],