fix Cauchy-Green tensor update

src/scheme/HyperelasticSolver.cpp

@@ -481,18 +481,19 @@ class HyperelasticSolverHandler::HyperelasticSolver final : public HyperelasticS
 
         Rd momentum_fluxes   = zero;
         double energy_fluxes = 0;
-        Rdxd cauchy_green_fluxes = zero;
+        Rdxd gradv           = zero;
         for (size_t R = 0; R < cell_nodes.size(); ++R) {
           const NodeId r = cell_nodes[R];
-          const Rdxd gradv = tensorProduct(ur[r], Cjr(j, R));
+          gradv += tensorProduct(ur[r], Cjr(j, R));
           momentum_fluxes += Fjr(j, R);
           energy_fluxes += dot(Fjr(j, R), ur[r]);
-          cauchy_green_fluxes += gradv * CG[j] + CG[j] * transpose(gradv);
         }
+        const Rdxd cauchy_green_fluxes = gradv * CG[j] + CG[j] * transpose(gradv);
         const double dt_over_Mj        = dt / (rho[j] * Vj[j]);
+        const double dt_over_Vj        = dt / Vj[j];
         new_u[j] += dt_over_Mj * momentum_fluxes;
         new_E[j] += dt_over_Mj * energy_fluxes;
-        new_CG[j] += dt_over_Mj * cauchy_green_fluxes;
+        new_CG[j] += dt_over_Vj * cauchy_green_fluxes;
         new_CG[j] += transpose(new_CG[j]);
         new_CG[j] *= 0.5;
       });