Fix #251

Since isnan() is only available in c++11, I prefer not to backport
this so v2.3.

At variance with older commit fba71ad4
I here only perform the check at the first step

src/vatom/COM.cpp

@@ -23,6 +23,7 @@
 #include "ActionRegister.h"
 #include "core/PlumedMain.h"
 #include "core/Atoms.h"
+#include <cmath>
 
 using namespace std;
 
@@ -69,6 +70,7 @@ class COM:
   public ActionWithVirtualAtom
 {
   bool nopbc;
+  bool first;
 public:
   explicit COM(const ActionOptions&ao);
   void calculate();
@@ -85,7 +87,8 @@ void COM::registerKeywords(Keywords& keys) {
 COM::COM(const ActionOptions&ao):
   Action(ao),
   ActionWithVirtualAtom(ao),
-  nopbc(false)
+  nopbc(false),
+  first(true)
 {
   vector<AtomNumber> atoms;
   parseAtomList("ATOMS",atoms);
@@ -110,6 +113,17 @@ void COM::calculate() {
   Vector pos;
   if(!nopbc) makeWhole();
   double mass(0.0);
+  if( first ) {
+    for(unsigned i=0; i<getNumberOfAtoms(); i++) {
+      if(isnan(getMass(i))) {
+        error(
+          "You are trying to compute a COM but masses are not known.\n"
+          "        If you are using plumed driver, please use the --mc option"
+        );
+      }
+    }
+    first=false;
+  }
   vector<Tensor> deriv(getNumberOfAtoms());
   for(unsigned i=0; i<getNumberOfAtoms(); i++) mass+=getMass(i);
   if( plumed.getAtoms().chargesWereSet() ) {