Fixed sign error in geometric term for HarmonicBond

mrdna/model/nonbonded.py

@@ -156,7 +156,7 @@ class HarmonicBond(HarmonicPotential):
             with np.errstate(divide='ignore',invalid='ignore'):
                 du = 2*0.58622592*np.log(dr+self.r0) * self.temperature/295
             du[np.logical_not(np.isfinite(du))] = 0
-            u = u-du
+            u = u+du
         return u