[Python] Implement 'allow_negative_pre_exponential_factor' property

interfaces/cython/cantera/reaction.pyx

@@ -297,6 +297,18 @@ cdef class ElementaryReaction(Reaction):
             cdef CxxElementaryReaction* r = <CxxElementaryReaction*>self.reaction
             r.rate = deref(rate.rate)
 
+    property allow_negative_pre_exponential_factor:
+        """
+        Get/Set whether the rate coefficient is allowed to have a negative
+        pre-exponential factor.
+        """
+        def __get__(self):
+            cdef CxxElementaryReaction* r = <CxxElementaryReaction*>self.reaction
+            return r.allow_negative_pre_exponential_factor
+        def __set__(self, allow):
+            cdef CxxElementaryReaction* r = <CxxElementaryReaction*>self.reaction
+            r.allow_negative_pre_exponential_factor = allow
+
 
 cdef class ThirdBodyReaction(ElementaryReaction):
     """

interfaces/cython/cantera/test/test_kinetics.py

@@ -657,6 +657,22 @@ class TestReaction(utilities.CanteraTest):
         self.assertNear(gas2.net_rates_of_progress[0],
                         self.gas.net_rates_of_progress[2])
 
+    def test_negative_A(self):
+        species = ct.Species.listFromFile('gri30.cti')
+        r = ct.ElementaryReaction('NH:1, NO:1', 'N2O:1, H:1')
+        r.rate = ct.Arrhenius(-2.16e13, -0.23, 0)
+
+        self.assertFalse(r.allow_negative_pre_exponential_factor)
+
+        with self.assertRaises(Exception):
+            gas = ct.Solution(thermo='IdealGas', kinetics='GasKinetics',
+                              species=species, reactions=[r])
+
+        r.allow_negative_pre_exponential_factor = True
+        gas = ct.Solution(thermo='IdealGas', kinetics='GasKinetics',
+                          species=species, reactions=[r])
+
+
     def test_thirdbody(self):
         r = ct.ThirdBodyReaction()
         r.reactants = {'O':1, 'H':1}