ImplicitSolve in Time dependent operator with Partial Assembly

[lmolin3]

Hi,
I've a working version of a Heat transfer solver extending ex16.
For now I'm using AssemblyLevel::LEGACY , but I made most of the changes required to include partial assembly (set assembly level, use supported preconditioners, include ConstrainedOperator for essential bcs, etc...).

I was wondering what would you suggest doing to adapt the code in the ImplicitSolve?
The operator is the same as ex16 plus a mass for the robin bcs T= M + dt K + dt RobinMass.
In all the examples I saw the operator T is always computed from the original assembled matrices, which in my case looks like:

{
               current_dt = dt;
               T = Add(1.0, *Mmat, 1.0, *Mmat_e); // T = M
               if (bcs->GetRobinBcs().size() > 0) // T = M + dt RobinMass
               {
                  auto tmp = ParAdd(Kmat, RobinMassmat);
                  T->Add(current_dt, *tmp);
                  delete tmp;
                  tmp = nullptr;
               }
               else
               {
                  T->Add(current_dt, *Kmat);
               }

               T_e = T->EliminateRowsCols(ess_tdof_list);
               T_solver.SetOperator(*T);
            }

I see that in ex9 Implicit time integration is not supported with partial assembly.

I thought about creating an Operator for du_dt = M^{-1}[-K(u + dtdu_dt) -RobinMass(u + dt*du_dt)] and solve inside the ImplicitSolve with a NewtonSolver, but even in that case the overloaded GetGradient would require access to the matrices M, K, RobinMass.

Thanks,
Leo

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