diff --git a/src/org/openscience/cdk/modeling/forcefield/IPotentialFunction.java b/src/org/openscience/cdk/modeling/forcefield/IPotentialFunction.java
index <HASH>..<HASH> 100644
--- a/src/org/openscience/cdk/modeling/forcefield/IPotentialFunction.java
+++ b/src/org/openscience/cdk/modeling/forcefield/IPotentialFunction.java
@@ -18,6 +18,7 @@ public interface IPotentialFunction {
 	GVector energyGradient = null;		//Gradient of the energy function in a 3xN point.
 	GMatrix energyHessian = null;
 	double[] forHessian = null;
+	int functionEvaluationNumber = 0;
 
 
 	/**
diff --git a/src/org/openscience/cdk/test/modeling/forcefield/TestPotentialFunction.java b/src/org/openscience/cdk/test/modeling/forcefield/TestPotentialFunction.java
index <HASH>..<HASH> 100644
--- a/src/org/openscience/cdk/test/modeling/forcefield/TestPotentialFunction.java
+++ b/src/org/openscience/cdk/test/modeling/forcefield/TestPotentialFunction.java
@@ -22,6 +22,7 @@ public class TestPotentialFunction implements IPotentialFunction {
 	double[] forHessian = null;
 	GMatrix order2ndErrorApproximateHessian = null;
 	double[] forOrder2ndErrorApproximateHessian = null;
+	int functionEvaluationNumber = 0;
 
 
 	/**
The variable functionEvaluationNumber was included in the PotentialFunction interface.

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