[Thermo] Fix silent failures in HP/UV/SV/SP convergence

In cases where the specified state was non-physical (i.e. corresponded to a
negative temperature), the iteration would exit when the temperature reached a
small enough value. Computing the error in temperature relative to the current
temperature avoids this problem.

Fixes #264

interfaces/cython/cantera/test/test_thermo.py

@@ -646,6 +646,12 @@ def test_setSV_lowT(self):
         self.assertNear(self.gas.v, 3 * v1)
         self.assertTrue(self.gas.T < self.gas.min_temp)
 
+    def test_setSV_low_invalid(self):
+        self.gas.TPX = 450, 1e5, 'H2:1.0, O2:0.4, AR:3'
+        self.gas.SV = 4600, None
+        with self.assertRaises(ct.CanteraError):
+            self.gas.SV = -1000, None
+
     def test_setSV_highT(self):
         """
         Set state in terms of (s,v) when the end temperature is above the
@@ -675,6 +681,16 @@ def test_setHP_lowT(self):
         self.assertNear(self.gas.P, p1)
         self.assertTrue(self.gas.T < self.gas.min_temp)
 
+    def test_setHP_low_invalid(self):
+        """
+        Set state in terms of (h,p) when the enthalpy would imply a negative
+        temperature
+        """
+
+        self.gas.TPX = 300, 101325, 'H2:1.0'
+        with self.assertRaises(ct.CanteraError):
+            self.gas.HP = -4e6, 101325
+
     def test_setHP_highT(self):
         """
         Set state in terms of (s,v) when the end temperature is above the

src/thermo/ThermoPhase.cpp

@@ -409,9 +409,9 @@ void ThermoPhase::setState_HPorUV(doublereal Htarget, doublereal p,
         // Convergence in H
         double Herr = Htarget - Hnew;
         double acpd = std::max(fabs(cpd), 1.0E-5);
-        double denom = std::max(fabs(Htarget), acpd * dTtol);
+        double denom = std::max(fabs(Htarget), acpd * Tnew);
         double HConvErr = fabs((Herr)/denom);
-        if (HConvErr < 0.00001 *dTtol || fabs(dt) < dTtol) {
+        if (HConvErr < 0.00001 * dTtol || fabs(dt/Tnew) < 0.00001 * dTtol) {
             return;
         }
     }
@@ -595,9 +595,9 @@ void ThermoPhase::setState_SPorSV(doublereal Starget, doublereal p,
         // Convergence in S
         double Serr = Starget - Snew;
         double acpd = std::max(fabs(cpd), 1.0E-5);
-        double denom = std::max(fabs(Starget), acpd * dTtol);
+        double denom = std::max(fabs(Starget), acpd * Tnew);
         double SConvErr = fabs((Serr * Tnew)/denom);
-        if (SConvErr < 0.00001 *dTtol || fabs(dt) < dTtol) {
+        if (SConvErr < 0.00001 * dTtol || fabs(dt/Tnew) < 0.00001 * dTtol) {
             return;
         }
     }