diff --git a/src/dfCanteraMixture/CanteraMixture.H b/src/dfCanteraMixture/CanteraMixture.H
index 7ecdc3b2..3556e4f5 100644
--- a/src/dfCanteraMixture/CanteraMixture.H
+++ b/src/dfCanteraMixture/CanteraMixture.H
@@ -117,7 +117,8 @@ public:
             const scalar& T
         ) const
         {
-            return CanteraGas_->meanMolecularWeight()/CanteraGas_->RT();
+            // for ideal gas, psi = CanteraGas_->density()/p = 1/RT
+            return CanteraGas_->density()/p;
         }
 
         scalar mu
diff --git a/src/dfChemistryModel/dfChemistryModel.C b/src/dfChemistryModel/dfChemistryModel.C
index 99016118..984c9364 100644
--- a/src/dfChemistryModel/dfChemistryModel.C
+++ b/src/dfChemistryModel/dfChemistryModel.C
@@ -383,8 +383,7 @@ void Foam::dfChemistryModel<ThermoType>::correctThermo()
 
         T_[celli] = CanteraGas_->temperature();
 
-        // meanMolecularWeight() kg/kmol    RT() Joules/kmol
-        psi_[celli] = CanteraGas_->meanMolecularWeight()/CanteraGas_->RT();
+        psi_[celli] = mixture_.psi(p_[celli],T_[celli]);
 
         mu_[celli] = mixture_.CanteraTransport()->viscosity(); // Pa-s
 
@@ -453,7 +452,7 @@ void Foam::dfChemistryModel<ThermoType>::correctThermo()
 
                 ph[facei] = CanteraGas_->enthalpy_mass();
 
-                ppsi[facei] = CanteraGas_->meanMolecularWeight()/CanteraGas_->RT();
+                ppsi[facei] = mixture_.psi(pp[facei],pT[facei]);
 
                 pmu[facei] = mixture_.CanteraTransport()->viscosity();
 
@@ -494,7 +493,7 @@ void Foam::dfChemistryModel<ThermoType>::correctThermo()
 
                 pT[facei] = CanteraGas_->temperature();
 
-                ppsi[facei] = CanteraGas_->meanMolecularWeight()/CanteraGas_->RT();
+                ppsi[facei] = mixture_.psi(pp[facei],pT[facei]);
 
                 pmu[facei] = mixture_.CanteraTransport()->viscosity();