diff --git a/mrdna/model/nonbonded.py b/mrdna/model/nonbonded.py
index 35ed22113877774093d143dc748c6a7dc20995b9..7e9c480673f458e073accfcdc3674441eebfd378 100644
--- a/mrdna/model/nonbonded.py
+++ b/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