diff --git a/src/thermophysicalModels/basic/psiThermo/ePsiThermo/ePsiThermo.H b/src/thermophysicalModels/basic/psiThermo/ePsiThermo/ePsiThermo.H
index 6952edc423..fd3071ff11 100644
--- a/src/thermophysicalModels/basic/psiThermo/ePsiThermo/ePsiThermo.H
+++ b/src/thermophysicalModels/basic/psiThermo/ePsiThermo/ePsiThermo.H
@@ -87,13 +87,13 @@ public:
 
     // Member functions
 
-        //- Return the compostion of the combustion mixture
+        //- Return the compostion of the mixture
         virtual basicMixture& composition()
         {
             return *this;
         }
 
-        //- Return the compostion of the combustion mixture
+        //- Return the compostion of the mixture
         virtual const basicMixture& composition() const
         {
             return *this;
diff --git a/src/thermophysicalModels/basic/psiThermo/hPsiThermo/hPsiThermo.H b/src/thermophysicalModels/basic/psiThermo/hPsiThermo/hPsiThermo.H
index c1d9b513a3..4eae7214a0 100644
--- a/src/thermophysicalModels/basic/psiThermo/hPsiThermo/hPsiThermo.H
+++ b/src/thermophysicalModels/basic/psiThermo/hPsiThermo/hPsiThermo.H
@@ -87,13 +87,13 @@ public:
 
     // Member functions
 
-        //- Return the compostion of the combustion mixture
+        //- Return the compostion of the mixture
         virtual basicMixture& composition()
         {
             return *this;
         }
 
-        //- Return the compostion of the combustion mixture
+        //- Return the compostion of the mixture
         virtual const basicMixture& composition() const
         {
             return *this;
diff --git a/src/thermophysicalModels/thermophysicalFunctions/APIfunctions/APIdiffCoefFunc/APIdiffCoefFunc.H b/src/thermophysicalModels/thermophysicalFunctions/APIfunctions/APIdiffCoefFunc/APIdiffCoefFunc.H
index b24d250d49..7f95e450e3 100644
--- a/src/thermophysicalModels/thermophysicalFunctions/APIfunctions/APIdiffCoefFunc/APIdiffCoefFunc.H
+++ b/src/thermophysicalModels/thermophysicalFunctions/APIfunctions/APIdiffCoefFunc/APIdiffCoefFunc.H
@@ -47,7 +47,7 @@ namespace Foam
 {
 
 /*---------------------------------------------------------------------------*\
-                           Class APIdiffCoefFunc Declaration
+                      Class APIdiffCoefFunc Declaration
 \*---------------------------------------------------------------------------*/
 
 class APIdiffCoefFunc
@@ -59,6 +59,9 @@ class APIdiffCoefFunc
         // API vapour mass diffusivity function coefficients
         scalar a_, b_, wf_, wa_;
 
+        // Helper variables
+        scalar alpha_, beta_;
+
 
 public:
 
@@ -74,7 +77,9 @@ public:
             a_(a),
             b_(b),
             wf_(wf),
-            wa_(wa)
+            wa_(wa),
+            alpha_(sqrt(1/wf_ + 1/wa_)),
+            beta_(sqr((cbrt(a_) + cbrt(b_))))
         {}
 
         //- Construct from Istream
@@ -83,21 +88,21 @@ public:
             a_(readScalar(is)),
             b_(readScalar(is)),
             wf_(readScalar(is)),
-            wa_(readScalar(is))
+            wa_(readScalar(is)),
+            alpha_(sqrt(1/wf_ + 1/wa_)),
+            beta_(sqr((cbrt(a_) + cbrt(b_))))
         {}
 
 
     // Member Functions
 
-        //- API vapour mass diffusivity function
+        //- API vapour mass diffusivity function using properties from
+        //  construction
         scalar f(scalar p, scalar T) const
         {
-            return
-               3.6059e-3*(pow(1.8*T, 1.75))*sqrt(1/wf_ + 1/wa_)
-               /(p*sqr((cbrt(a_) + cbrt(b_))));
+            return 3.6059e-3*(pow(1.8*T, 1.75))*alpha_/(p*beta_);
         }
 
-
         //- Write the function coefficients
         void writeData(Ostream& os) const
         {