Update RPMD enforcePeriodicBox to support triclinic unit cells

plugins/rpmd/openmmapi/src/RPMDIntegrator.cpp

@@ -128,21 +128,15 @@ State RPMDIntegrator::getState(int copy, int types, bool enforcePeriodicBox, int
             center *= 1.0/molecules[i].size();
 
             // Find the displacement to move it into the first periodic box.
-
-            int xcell = (int) floor(center[0]/periodicBoxSize[0][0]);
-            int ycell = (int) floor(center[1]/periodicBoxSize[1][1]);
-            int zcell = (int) floor(center[2]/periodicBoxSize[2][2]);
-            double dx = xcell*periodicBoxSize[0][0];
-            double dy = ycell*periodicBoxSize[1][1];
-            double dz = zcell*periodicBoxSize[2][2];
+            Vec3 diff;
+            diff -= periodicBoxSize[0]*static_cast<int>(center[0]/periodicBoxSize[0][0]);
+            diff -= periodicBoxSize[1]*static_cast<int>(center[1]/periodicBoxSize[1][1]);
+            diff -= periodicBoxSize[2]*static_cast<int>(center[2]/periodicBoxSize[2][2]);
 
             // Translate all the particles in the molecule.
-
             for (int j = 0; j < (int) molecules[i].size(); j++) {
                 Vec3& pos = positions[molecules[i][j]];
-                pos[0] -= dx;
-                pos[1] -= dy;
-                pos[2] -= dz;
+                pos -= diff;
             }
         }