document and print warning when energy is tallied with momentum removal

doc/fix_langevin.txt

@@ -217,6 +217,10 @@ the particles.  As described below, this energy can then be printed
 out or added to the potential energy of the system to monitor energy
 conservation.
 
+IMPORTANT NOTE: this accumulated energy does NOT include kinetic
+energy removed by the {zero} flag. LAMMPS will print a warning when
+both options are active.
+
 The keyword {zero} can be used to eliminate drift due to the
 thermostat. Because the random forces on different atoms are
 independent, they do not sum exactly to zero.  As a result, this fix

src/fix_langevin.cpp

@@ -174,6 +174,8 @@ FixLangevin::FixLangevin(LAMMPS *lmp, int narg, char **arg) :
     }
   }
 
+  if (tallyflag && zeroflag && comm->me == 0)
+    error->warning(FLERR,"Energy tally does not account for 'zero yes'");
 }
 
 /* ---------------------------------------------------------------------- */

src/fix_langevin.h

@@ -103,6 +103,12 @@ E: Fix langevin period must be > 0.0
 
 The time window for temperature relaxation must be > 0
 
+W: Energy tally does not account for 'zero yes'
+
+The energy removed by using the 'zero yes' flag is not accounted
+for in the energy tally and thus energy conservation cannot be
+monitored in this case.
+
 E: Fix langevin omega requires atom style sphere
 
 Self-explanatory.