diff --git a/diffusivityModel/N64/N64.C b/diffusivityModel/N64/N64.C
index dfeec40..9bfcbf9 100644
--- a/diffusivityModel/N64/N64.C
+++ b/diffusivityModel/N64/N64.C
@@ -47,6 +47,10 @@ const Foam::scalar Foam::N64::a22[(M+1)*(M+2)/2] = {
 
 Foam::scalar Foam::N64::vec[(M+1)*(M+2)/2] = { 0.0 };
 
+const Foam::scalar Foam::N64::digamma3 = 0.922784335098467139393487909917597568957840664060;
+
+const Foam::scalar Foam::N64::trigamma3 = sqr(Ion::pi) / 6. - 5./4.;
+
 // * * * * * * * * * * * * * Static Member Functions * * * * * * * * * * * * //
 
 
diff --git a/diffusivityModel/N64/N64.H b/diffusivityModel/N64/N64.H
index 35da13c..d572219 100644
--- a/diffusivityModel/N64/N64.H
+++ b/diffusivityModel/N64/N64.H
@@ -99,6 +99,21 @@ class N64
         static const scalar a22[(M+1)*(M+2)/2];
         static scalar vec[(M+1)*(M+2)/2];
 
+        //- Construct null
+        static constexpr scalar K1 = 1.767;
+
+        //- Construct null
+        static constexpr scalar K2 = 0.720;
+
+        //- Construct null
+        static constexpr scalar kappa = 0.0095;
+
+        //- Construct null
+        static const scalar digamma3;
+
+        //- Construct null
+        static const scalar trigamma3;
+
 
     // Private Member Functions
 
@@ -155,15 +170,6 @@ public:
 
     // Static data members
 
-        //- Construct null
-        static constexpr scalar K1 = 1.767;
-
-        //- Construct null
-        static constexpr scalar K2 = 0.720;
-
-        //- Construct null
-        static constexpr scalar kappa = 0.0095;
-
 
     // Constructors