[Kinetics] Prevent positive feedback in negative species concentrations

For binary (or higher order) reactions, multiple negative (but small, i.e. with
respect to integration tolerances) species concentrations should not produce
positive reaction rates that would drive those concentrations to even more
negative values.

include/cantera/kinetics/StoichManager.h

@@ -220,7 +220,11 @@ class C2
     }
 
     void multiply(const doublereal* S, doublereal* R) const {
-        R[m_rxn] *= S[m_ic0] * S[m_ic1];
+        if (S[m_ic0] < 0 && S[m_ic1] < 0) {
+            R[m_rxn] = 0;
+        } else {
+            R[m_rxn] *= S[m_ic0] * S[m_ic1];
+        }
     }
 
     void incrementReaction(const doublereal* S, doublereal* R) const {
@@ -281,7 +285,12 @@ class C3
     }
 
     void multiply(const doublereal* S, doublereal* R) const {
-        R[m_rxn] *= S[m_ic0] * S[m_ic1] * S[m_ic2];
+        if ((S[m_ic0] < 0 && (S[m_ic1] < 0 || S[m_ic2] < 0)) ||
+            (S[m_ic1] < 0 && S[m_ic2] < 0)) {
+            R[m_rxn] = 0;
+        } else {
+            R[m_rxn] *= S[m_ic0] * S[m_ic1] * S[m_ic2];
+        }
     }
 
     void incrementReaction(const doublereal* S, doublereal* R) const {
@@ -357,12 +366,19 @@ class C_AnyN
 
     void multiply(const doublereal* input, doublereal* output) const {
         doublereal oo;
+        int neg_count = 0;
         for (size_t n = 0; n < m_n; n++) {
             oo = m_order[n];
             if (oo != 0.0) {
+                if (input[m_ic[n]] < 0) {
+                    neg_count++;
+                }
                 output[m_rxn] *= ppow(input[m_ic[n]], oo);
             }
         }
+        if (neg_count > 1) {
+            output[m_rxn] = 0;
+        }
     }
 
     void incrementSpecies(const doublereal* input,