diff --git AmberTools/src/mmpbsa_py/MMPBSA_mods/amberoutputs.py AmberTools/src/mmpbsa_py/MMPBSA_mods/amberoutputs.py
index 23b5d82..f1f963d 100644
--- AmberTools/src/mmpbsa_py/MMPBSA_mods/amberoutputs.py
+++ AmberTools/src/mmpbsa_py/MMPBSA_mods/amberoutputs.py
@@ -2311,7 +2311,7 @@ class NMODEout:
    def _tot_stdevs(self):
       """ Returns the standard deviations of the total based on whether or not we were able to
           calculate all frames or not. Use a sample stdev if we were, sum-of-squares if we weren't """
-      if len(self.com_data) != len(self.rec_data) or len(self.com_data) != len(self.lig_data):
+      if len(self.com_data[0]) != len(self.rec_data[0]) or len(self.com_data[0]) != len(self.lig_data[0]):
          return math.sqrt(abs(_stdev(self.com_data[0]) ** 2 + _stdev(self.rec_data[0]) ** 2 + _stdev(self.lig_data[0]) ** 2))
 
       sum = sum2 = 0
@@ -2319,8 +2319,8 @@ class NMODEout:
          sum += self.com_data[0][i] - self.rec_data[0][i] - self.lig_data[0][i]
          sum2 += (self.com_data[0][i] - self.rec_data[0][i] - self.lig_data[0][i]) ** 2
 
-      avg = sum / len(self.com_data)
-      return math.sqrt(abs(sum2 / len(self.com_data) - avg * avg))
+      avg = sum / len(self.com_data[0])
+      return math.sqrt(abs(sum2 / len(self.com_data[0]) - avg * avg))
 
 #==============================