Merge pull request #1519 from peastman/water

Fixed an exception in addSolvent()

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

@@ -345,6 +345,9 @@ class Modeller(object):
         elif padding is not None:
             if is_quantity(padding):
                 padding = padding.value_in_unit(nanometer)
+            if len(self.positions) == 0:
+                maxSize = 0
+            else:
                 maxSize = max(max((pos[i] for pos in self.positions))-min((pos[i] for pos in self.positions)) for i in range(3))
                 maxSize = maxSize.value_in_unit(nanometer)
             box = (maxSize+2*padding)*Vec3(1, 1, 1)