Fix #929 Raise error for non-physical terms (#1086)

Raise ValueError for issue
[929](#929) when terms
are unexpected because only one- and two-body operators are present in
molecular Hamiltonian (See Eq. (4) - (7) in
https://www.nature.com/articles/ncomms5213)

Author: SamKChang

src/openfermion/measurements/equality_constraint_projection.py

@@ -51,6 +51,11 @@ def linearize_term(term, n_orbitals):
         r = term[2][0]
         s = term[3][0]
         return 1 + n_orbitals**2 + p + q * n_orbitals + r * n_orbitals**2 + s * n_orbitals**3
+    raise ValueError(
+        'Expect one-body (2 terms) or two-body (4 terms) operator but got {} terms. '.format(
+            len(term)
+        )
+    )
 
 
 def unlinearize_term(index, n_orbitals):

src/openfermion/measurements/equality_constraint_projection_test.py

@@ -14,6 +14,7 @@
 import unittest
 import numpy
 
+from openfermion.ops.operators import FermionOperator
 from openfermion.chem import MolecularData
 from openfermion.config import DATA_DIRECTORY
 from openfermion.transforms.opconversions import get_fermion_operator
@@ -59,6 +60,11 @@ def test_linearize_term(self):
             self.assertFalse(index in past_terms)
             past_terms.add(index)
 
+    def test_error_with_non_physical_term(self):
+        non_physical_operator = FermionOperator((0, 1))
+        with self.assertRaises(ValueError):
+            linearize_term(non_physical_operator.terms, self.n_orbitals)
+
     def test_unlinearize_term_consistency(self):
         for term, _ in self.fermion_hamiltonian.terms.items():
             index = linearize_term(term, self.n_orbitals)