ZeroDivisionError Fix to Metadynamics Bias (#3636)

* ZeroDivisionError Fix to Metadynamics Bias

* Corrected Error Message

wrappers/python/openmm/app/metadynamics.py

@@ -105,8 +105,8 @@ class Metadynamics(object):
             temperature = temperature*unit.kelvin
         if not unit.is_quantity(height):
             height = height*unit.kilojoules_per_mole
-        if biasFactor < 1.0:
-            raise ValueError('biasFactor must be >= 1')
+        if biasFactor <= 1.0:
+            raise ValueError('biasFactor must be > 1')
         if (saveFrequency is None and biasDir is not None) or (saveFrequency is not None and biasDir is None):
             raise ValueError('Must specify both saveFrequency and biasDir')
         if saveFrequency is not None and (saveFrequency < frequency or saveFrequency%frequency != 0):