diff --git a/mrdna/segmentmodel.py b/mrdna/segmentmodel.py
index 80436df0192b94cff995566b429bcc8da72be6bf..13a5d74462259059836791a4621a9f1402c074d5 100644
--- a/mrdna/segmentmodel.py
+++ b/mrdna/segmentmodel.py
@@ -2075,7 +2075,7 @@ class SegmentModel(ArbdModel):
         if self.DEBUG: print("Adding intrahelical angle potentials")
         for b1,b2,b3 in self._get_intrahelical_angle_beads():
             ## TODO: could be slightly smarter about sep
-            sep = 0.5*b1.num_nt+b2.num_nt+0.5*b3.num_nt
+            sep = 0.5*(0.5*b1.num_nt+b2.num_nt+0.5*b3.num_nt)
             parent = self._getParent(b1,b2,b3)
 
             kT = 0.58622522         # kcal/mol