Allow empty operator initialization

Anthony-Gandon · Jun 3, 2021 · 3692340 · 3692340
1 parent af1691d
commit 3692340
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Showing 3 changed files with 20 additions and 11 deletions.
diff --git a/qiskit_nature/operators/second_quantization/fermionic_op.py b/qiskit_nature/operators/second_quantization/fermionic_op.py
@@ -177,6 +177,14 @@ def __init__(
         if isinstance(data, str):
             data = [(data, 1)]
 
+        if data == []:
+            if register_length is None:
+                raise TypeError(
+                    "If you want to initialize an empty operator, you _must_ specify the length of "
+                    "the register!"
+                )
+            data = [("I_0", 0)]
+
         if not all(
             isinstance(label, str) and isinstance(coeff, (int, float, complex))
             for label, coeff in data

diff --git a/qiskit_nature/operators/second_quantization/vibrational_op.py b/qiskit_nature/operators/second_quantization/vibrational_op.py
@@ -119,6 +119,16 @@ def __init__(
         if isinstance(data, str):
             data = [(data, 1)]
 
+        if isinstance(num_modals, int):
+            num_modals = [num_modals] * num_modes
+
+        self._num_modes = num_modes
+        self._num_modals = num_modals
+        self._register_length = sum(self._num_modals)
+
+        if data == []:
+            data = [("I" * self._register_length, 0)]
+
         if not all(
             isinstance(label, str) and isinstance(coeff, (int, float, complex))
             for label, coeff in data
@@ -128,13 +138,6 @@ def __init__(
         self._coeffs: np.ndarray
         self._labels: List[str]
 
-        if isinstance(num_modals, int):
-            num_modals = [num_modals] * num_modes
-
-        self._num_modes = num_modes
-        self._num_modals = num_modals
-        self._register_length = sum(self._num_modals)
-
         labels, coeffs = zip(*data)
         self._coeffs = np.array(coeffs, np.complex128)
 

diff --git a/qiskit_nature/properties/electronic_integrals.py b/qiskit_nature/properties/electronic_integrals.py
@@ -92,10 +92,8 @@ def to_second_q_op(self) -> FermionicOp:
         """TODO."""
         base_ops_labels = self._create_base_ops()
 
-        # TODO: allow an empty list as argument to FermionicOp
-        fac = 2 if self._basis != Basis.SO else 1
-        initial_label_with_ceoff = ("I" * fac * len(self._matrices[0]), 0)
-        base_ops_labels.append(initial_label_with_ceoff)
+        if base_ops_labels == []:
+            return FermionicOp([], register_length=len(self._matrices[0]))
 
         return FermionicOp(base_ops_labels)