bugfix for virial tally for improper style umbrella from Steven Vandenbrande (U Gent)

this closes #182

(cherry picked from commit 4921dc18)

src/MOLECULE/improper_umbrella.cpp

@@ -231,9 +231,26 @@ void ImproperUmbrella::compute(int eflag, int vflag)
       f[i4][2] += f4[2]*a;
     }
 
-    if (evflag)
+    if (evflag) {
+
+      // correct 4-body geometry for virial tally
+
+      vb1x = x[i1][0] - x[i2][0];
+      vb1y = x[i1][1] - x[i2][1];
+      vb1z = x[i1][2] - x[i2][2];
+
+      vb2x = x[i3][0] - x[i2][0];
+      vb2y = x[i3][1] - x[i2][1];
+      vb2z = x[i3][2] - x[i2][2];
+
+      vb3x = x[i4][0] - x[i3][0];
+      vb3y = x[i4][1] - x[i3][1];
+      vb3z = x[i4][2] - x[i3][2];
+
+
       ev_tally(i1,i2,i3,i4,nlocal,newton_bond,eimproper,f1,f3,f4,
                vb1x,vb1y,vb1z,vb2x,vb2y,vb2z,vb3x,vb3y,vb3z);
+    }
   }
 }
 

src/USER-OMP/improper_umbrella_omp.cpp

@@ -254,8 +254,24 @@ void ImproperUmbrellaOMP::eval(int nfrom, int nto, ThrData * const thr)
       f[i4].z += f4[2]*a;
     }
 
-    if (EVFLAG)
+    if (EVFLAG) {
+
+      // get correct 4-body geometry for virial tally
+
+      vb1x = x[i1].x - x[i2].x;
+      vb1y = x[i1].y - x[i2].y;
+      vb1z = x[i1].z - x[i2].z;
+
+      vb2x = x[i3].x - x[i2].x;
+      vb2y = x[i3].y - x[i2].y;
+      vb2z = x[i3].z - x[i2].z;
+
+      vb3x = x[i4].x - x[i3].x;
+      vb3y = x[i4].y - x[i3].y;
+      vb3z = x[i4].z - x[i3].z;
+
       ev_tally_thr(this,i1,i2,i3,i4,nlocal,NEWTON_BOND,eimproper,f1,f3,f4,
                    vb1x,vb1y,vb1z,vb2x,vb2y,vb2z,vb3x,vb3y,vb3z,thr);
+    }
   }
 }