Merge pull request #17 from peastman/master

Bug fix to Modeller

wrappers/python/simtk/openmm/app/modeller.py

@@ -38,7 +38,8 @@ from simtk.openmm.app.forcefield import HAngles, _createResidueSignature, _match
 from simtk.openmm.app.topology import Residue
 from simtk.openmm.vec3 import Vec3
 from simtk.openmm import System, Context, NonbondedForce, VerletIntegrator, LocalEnergyMinimizer
-from simtk.unit import nanometer, molar, elementary_charge, amu, gram, liter, degree, sqrt, acos, is_quantity, dot, norm, sum
+from simtk.unit import nanometer, molar, elementary_charge, amu, gram, liter, degree, sqrt, acos, is_quantity, dot, norm
+import simtk.unit as unit
 import element as elem
 import os
 import random
@@ -914,7 +915,7 @@ class Modeller(object):
                                 # and hope that energy minimization will fix it.
                                 
                                 knownPositions = [x for x in templateAtomPositions if x is not None]
-                                position = sum(knownPositions)/len(knownPositions)
+                                position = unit.sum(knownPositions)/len(knownPositions)
                             newPositions.append(position)
         for bond in self.topology.bonds():
             if bond[0] in newAtoms and bond[1] in newAtoms: