make the recentering displacement available as global scalar/vector

doc/fix_recenter.txt

@@ -91,9 +91,20 @@ to tether the molecule in place.
 
 No information about this fix is written to "binary restart
 files"_restart.html.  None of the "fix_modify"_fix_modify.html options
-are relevant to this fix.  No global or per-atom quantities are stored
-by this fix for access by various "output
-commands"_Section_howto.html#howto_15.  No parameter of this fix can
+are relevant to this fix.
+
+This fix computes a global scalar which can be accessed by various
+"output commands"_Section_howto.html#howto_15.  The scalar is the
+distance the group is moved by fix recenter.
+
+This fix also computes global 3-vector which can be accessed by
+various "output commands"_Section_howto.html#howto_15.  The 3
+quantities in the vector are xyz components of displacement applied
+by to the group of atoms by the fix.
+
+The scalar and vector values calculated by this fix are "extensive".
+
+No parameter of this fix can
 be used with the {start/stop} keywords of the "run"_run.html command.
 This fix is not invoked during "energy minimization"_minimize.html.
 

src/fix_recenter.cpp

@@ -42,6 +42,15 @@ FixRecenter::FixRecenter(LAMMPS *lmp, int narg, char **arg) :
   xcom = ycom = zcom = 0.0;
   xflag = yflag = zflag = 1;
   xinitflag = yinitflag = zinitflag = 0;
+  shift[0] = shift[1] = shift[2] = 0.0;
+  distance = 0.0;
+  scalar_flag = 1;
+  vector_flag = 1;
+  size_vector = 3;
+  extscalar = 1;
+  extvector = 1;
+
+/* ---------------------------------------------------------------------- */
 
   if (strcmp(arg[3],"NULL") == 0) xflag = 0;
   else if (strcmp(arg[3],"INIT") == 0) xinitflag = 1;
@@ -177,10 +186,30 @@ void FixRecenter::initial_integrate(int vflag)
   int *mask = atom->mask;
   int nlocal = atom->nlocal;
 
+  shift[0] = xflag ? (xtarget - xcm[0]) : 0.0;
+  shift[1] = yflag ? (ytarget - xcm[1]) : 0.0;
+  shift[2] = zflag ? (ztarget - xcm[2]) : 0.0;
+  distance = sqrt(shift[0]*shift[0] + shift[1]*shift[1] + shift[2]*shift[2]);
+
   for (int i = 0; i < nlocal; i++)
     if (mask[i] & group2bit) {
-      if (xflag) x[i][0] += xtarget - xcm[0];
-      if (yflag) x[i][1] += ytarget - xcm[1];
-      if (zflag) x[i][2] += ztarget - xcm[2];
+      x[i][0] += shift[0];
+      x[i][1] += shift[1];
+      x[i][2] += shift[2];
     }
 }
+
+/* ---------------------------------------------------------------------- */
+
+double FixRecenter::compute_scalar()
+{
+  return distance;
+}
+
+/* ---------------------------------------------------------------------- */
+
+double FixRecenter::compute_vector(int n)
+{
+  return shift[n];
+}
+

src/fix_recenter.h

@@ -30,12 +30,14 @@ class FixRecenter : public Fix {
   int setmask();
   void init();
   void initial_integrate(int);
+  double compute_scalar();
+  double compute_vector(int);
 
  private:
   int group2bit,scaleflag;
   int xflag,yflag,zflag;
   int xinitflag,yinitflag,zinitflag;
-  double xcom,ycom,zcom,xinit,yinit,zinit,masstotal;
+  double xcom,ycom,zcom,xinit,yinit,zinit,masstotal,distance,shift[3];
 };
 
 }