diff --git a/mrdna/coords.py b/mrdna/coords.py
index b74906da417e0b7cc787be05fbdb422e24609705..41628e83501c17098094fc12b7d4d4e6594ebcb6 100644
--- a/mrdna/coords.py
+++ b/mrdna/coords.py
@@ -150,7 +150,7 @@ def quaternion_from_matrix( R ):
 
 def rotationAboutAxis(axis,angle, normalizeAxis=True):
     if normalizeAxis: axis = axis / np.linalg.norm(axis)
-    angle *= 0.5 * np.pi/180
+    angle = angle * 0.5 * np.pi/180
     cos = np.cos( angle )
     sin = np.sin( angle )
     q = [cos] + [sin*x for x in axis]
diff --git a/mrdna/segmentmodel.py b/mrdna/segmentmodel.py
index 068271454fe2eb5c256956f68e5a5f4cf4ae3865..61ddfca9e9ddfa57e7459941503bbdebdbfab7cf 100644
--- a/mrdna/segmentmodel.py
+++ b/mrdna/segmentmodel.py
@@ -420,7 +420,7 @@ class Segment(ConnectableElement, Group):
             ids = list(filter(lambda i: position_filter(r[i,:]), ids))
         return ids
 
-    def translate(self, translation_vector, about=None, position_filter=None, contour_filter=None):
+    def translate(self, translation_vector, position_filter=None, contour_filter=None):
         dr = np.array(translation_vector)
         tck, u = self.position_spline_params
         r = self.contour_to_position(u)
@@ -429,7 +429,7 @@ class Segment(ConnectableElement, Group):
 
         ## Translate
         r[ids,:] = r[ids,:] + dr[np.newaxis,:]
-        self.set_position_splines(r,u)
+        self.set_splines(u,r)
 
     def rotate(self, rotation_matrix, about=None, position_filter=None, contour_filter=None):
         tck, u = self.position_spline_params
@@ -445,16 +445,16 @@ class Segment(ConnectableElement, Group):
             ## TODO: do this more efficiently
             r[ids,:] = np.array([rotation_matrix.dot(r[i,:]-dr) + dr for  i in range(r.shape[0])])
 
-        self.set_position_splines(r,u)
+        self.set_splines(u,r)
 
         if self.quaternion_spline_params is not None:
             ## TODO: performance: don't shift between quaternion and matrix representations so much
             tck, u = self.quaternion_spline_params
-            orientations = [self.contour_to_orientation(v)) for v in u]
+            orientations = [self.contour_to_orientation(v) for v in u]
             for i in ids:
                 orientations[i,:] = rotation_matrix.dot(orientations[i])
             quats = [quaternion_from_matrix(o) for o in orientations]
-            self.set_orientation_splines(quats,u)
+            self.set_orientation_splines(u, quats)
 
     def _set_splines_from_ends(self):
         self.quaternion_spline_params = None
@@ -1386,6 +1386,7 @@ class SegmentModel(ArbdModel):
 
         self._bonded_potential = dict() # cache for bonded potentials
         self._generate_strands()
+        self.potentials = []
         self._generate_bead_model( max_basepairs_per_bead, max_nucleotides_per_bead, local_twist, escapable_twist)
 
         self.useNonbondedScheme( nbDnaScheme )
@@ -1540,6 +1541,26 @@ class SegmentModel(ArbdModel):
         print("WARNING: called deprecated command '_update_segment_positions; use 'update_splines' instead")
         return self.update_splines(bead_coordinates)
 
+    ## Operations on spline coordinates
+    def translate(self, translation_vector, position_filter=None):
+        for s in self.segments:
+            s.translate(translation_vector, position_filter=position_filter)
+        
+    def rotate(self, rotation_matrix, about=None, position_filter=None):
+        for s in self.segments:
+            s.rotate(rotation_matrix, about=about, position_filter=position_filter)
+
+    def get_center(self, include_ssdna=False):
+        if include_ssdna:
+            segments = self.segments
+        else:
+            segments = list(filter(lambda s: isinstance(s,DoubleStrandedSegment), 
+                                   self.segments))
+        centers = [s.get_center() for s in segments]
+        weights = [s.num_nt*2 if isinstance(s,DoubleStrandedSegment) else s.num_nt for s in segments]
+        # centers,weights = [np.array(a) for a in (centers,weights)]
+        return np.average( centers, axis=0, weights=weights)
+
     def update_splines(self, bead_coordinates):
         """ Set new function for each segments functions
         contour_to_position and contour_to_orientation """
@@ -1626,14 +1647,8 @@ class SegmentModel(ArbdModel):
             #     print("  {}[{:03f}]: {:0.3f}".format(b.parent.name, b.contour_position, c) )
 
 
-            ret = interpolate.splprep( positions, u=contours, s=0, k=1, full_output=1 )
-            tck = ret[0][0]
+            tck, u = interpolate.splprep( positions, u=contours, s=0, k=1 )
 
-            ret = interpolate.splprep( positions, u=contours, s=0, k=1, full_output=1 )
-            tck2 = ret[0][0]
-
-            assert( tck[0][0] == tck2[0][0] )
-            
             # if len(beads) < 8:
             #     ret = interpolate.splprep( positions, u=contours, s=0, k=1, full_output=1 )
             #     tck = ret[0][0]
@@ -1652,7 +1667,7 @@ class SegmentModel(ArbdModel):
             #         if ret[2] > 0:
             #             pdb.set_trace()
 
-            s.position_spline_params = (tck,ret[1][0])
+            s.position_spline_params = (tck,u)
 
             """ Get twist """
             cb = [e for e in zip(contours,beads) if 'orientation_bead' in e[1].__dict__]