diff --git a/c/interaction/interaction.c b/c/interaction/interaction.c
index 9048dee..efb50a0 100644
--- a/c/interaction/interaction.c
+++ b/c/interaction/interaction.c
@@ -134,7 +134,7 @@ static PyTypeObject interaction_UFuncWrapperType
 {
 	PyTypeObject_HEAD_INIT(NULL, 0)
 	.tp_name = "brown.interaction.UFuncWrapper",
-	.tp_doc = "A wrapper that wraps the ufuncs for interaction and force, storing the coeficients",
+	.tp_doc = "A wrapper that wraps the ufuncs for interaction and force, storing the coefficients",
 	.tp_basicsize = sizeof(interaction_UFuncWrapper),
 	.tp_itemsize = 0,
 	.tp_flags = Py_TPFLAGS_DEFAULT,
@@ -142,6 +142,12 @@ static PyTypeObject interaction_UFuncWrapperType
 
 };
 
+
+static char interaction_types[] = 
+	{ NPY_FLOAT, NPY_FLOAT, NPY_FLOAT, NPY_FLOAT, NPY_FLOAT, NPY_FLOAT};
+static char force_types[] =
+	{ NPY_FLOAT, NPY_FLOAT};
+
 static int
 interaction_UFuncWrapper_init
 	( interaction_UFuncWrapper * self
@@ -152,6 +158,8 @@ interaction_UFuncWrapper_init
 	// 1: interaction2D
 	char type;
 	PyObject * coefficients;
+	int i;
+	PyObject * this_coefficient;
 
 	if(!PyArgs_ParseTupleAndKeywords(args, kwds, "BO!", &type, &PyList_Type, &coefficients))
 	{
@@ -164,35 +172,76 @@ interaction_UFuncWrapper_init
 		return -1;
 	}
 
+	// copy the coefficients.
+	for(i = 0; i < 19; i++)
+	{
+		this_coefficient = PyList_GetItem(coefficients, i);
+		// XXX: PyFloat_AsDouble might call python code,
+		// so make sure that nothing bad can happen.
+		Py_INCREF(this_coefficient);
+		self->coefficients[i] = PyFloat_AsDouble(this_coefficient);
+		Py_DECREF(this_coefficient);
+		if(PyErr_Occurred())
+		{
+			return -1;
+		}
+	}
 
+	switch(type)
+	{
+		case 0:
+		{
+			self->ufunc = PyUFunc_FromFuncAndData(
+				force_funcs
+				, self->coefficients
+				, force_types
+				, 1
+				, 2
+				, 1
+				, PyUFunc_None
+				, "force_function"
+				, "computes the scalar force between two particles with given coefficients"
+				, 0);
+			break;
+		}
+		case 1:
+		{
+			self->ufunc = PyUFunc_FromFuncAndData(
+				interaction_funcs
+				, interaction_data
+				, interaction_types
+				, 1
+				, 5
+				, 2
+				, PyUFunc_None
+				, "interaction2D"
+				, "Update the momenta according to the given coefficients and positions"
+				, 0);
+			break;
+		}
+		default:
+		{
+			PyErr_SetString(PyExc_ValueError, "unknown ufunc type, must be 0 or 1");
+			return -1
+		}
+
+	}
 }
 
 
 static PyObject *
 interaction_UFuncWrapper_call
-	(interaction_UFuncWrapper * self,
-	 PyObject * data)
+	(interaction_UFuncWrapper * self
+	 , PyObject * args
+	 , PyObject * kwargs)
 {
-	return 
+	return PyObject_Call(self->ufunc, args, kwargs);
 }
 
 static PyMethodDef InteractionMethods[] = {
 	{NULL, NULL, 0, NULL}
 };
 
-// FIXME
-PyUFuncGenericFunction interaction_funcs[] = 
-	{ &interaction_ufunc_float2D};
-PyUFuncGenericFunction force_funcs[] = 
-	{ &interaction_ufunc_force};
-
-static char interaction_types[] = 
-	{ NPY_FLOAT, NPY_FLOAT, NPY_FLOAT, NPY_FLOAT, NPY_FLOAT, NPY_FLOAT, NPY_FLOAT};
-static char force_types[] =
-	{ NPY_FLOAT, NPY_FLOAT, NPY_FLOAT};
-
-static void *interaction_data[] = {NULL};
-static void *force_data[] = {NULL};
 
 static struct PyModuleDef moduledef = {
 	PyModuleDef_HEAD_INIT
@@ -223,29 +272,7 @@ PyInit_interaction(void)
 	import_ufunc();
 
 	ufunc_interaction = PyUFunc_FromFuncAndDataAndSignature(
-			interaction_funcs
-			, interaction_data
-			, interaction_types
-			, 1
-			, 5
-			, 2
-			, PyUFunc_None
-			, "interaction2D"
-			, "Update the momenta according to the given coefficients and positions"
-			, 0
-			, "(n),(n),(n),(n),(19)->(n),(n)");
 	ufunc_force = PyUFunc_FromFuncAndDataAndSignature(
-			force_funcs
-			, force_data
-			, force_types
-			, 1
-			, 2
-			, 1
-			, PyUFunc_None
-			, "force_function"
-			, "computes the scalar force between two particles with given coefficients"
-			, 0
-			, "(n),(19)->(n)");
 
 	dct = PyModule_GetDict(module);
 	PyDict_SetItemString(dct, "interaction2D", ufunc_interaction);