mixed_poisson.cpp

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/**
 * \file mixed_poisson.cpp
 * \example mixed_poisson.cpp
 *
 * MixedPoisson intended to show how to solve mixed formulation of the Dirichlet
 * problem for the Poisson equation using error indicators and p-adaptivity
 *
 */
 
#include <MoFEM.hpp>
 
using namespace MoFEM;
 
static char help[] = "...\n\n";
 
#include <BasicFiniteElements.hpp>
 
using DomainEle = PipelineManager::FaceEle;
using DomainEleOp = DomainEle::UserDataOperator;
using EntData = EntitiesFieldData::EntData;
 
using OpHdivHdiv = FormsIntegrators<DomainEleOp>::Assembly<PETSC>::BiLinearForm<
    GAUSS>::OpMass<3, 2>;
using OpHdivU = FormsIntegrators<DomainEleOp>::Assembly<PETSC>::BiLinearForm<
    GAUSS>::OpMixDivTimesScalar<2>;
using OpDomainSource = FormsIntegrators<DomainEleOp>::Assembly<
    PETSC>::LinearForm<GAUSS>::OpSource<1, 1>;
 
inline double sqr(double x) { return x * x; }
 
struct MixedPoisson {
 
  MixedPoisson(MoFEM::Interface &m_field) : mField(m_field) {}
  MoFEMErrorCode runProblem();
 
private:
  MoFEM::Interface &mField;
  Simple *simpleInterface;
 
  Range domainEntities;
  double totErrorIndicator;
  double maxErrorIndicator;
 
  double thetaParam;
  double indicTolerance;
  int initOrder;
 
  //! [Analytical function]
  static double analyticalFunction(const double x, const double y,
                                   const double z) {
    return exp(-100. * (sqr(x) + sqr(y))) * cos(M_PI * x) * cos(M_PI * y);
  }
  //! [Analytical function]
 
  //! [Analytical function gradient]
  static VectorDouble analyticalFunctionGrad(const double x, const double y,
                                             const double z) {
    VectorDouble res;
    res.resize(2);
    res[0] = -exp(-100. * (sqr(x) + sqr(y))) *
             (200. * x * cos(M_PI * x) + M_PI * sin(M_PI * x)) * cos(M_PI * y);
    res[1] = -exp(-100. * (sqr(x) + sqr(y))) *
             (200. * y * cos(M_PI * y) + M_PI * sin(M_PI * y)) * cos(M_PI * x);
    return res;
  }
  //! [Analytical function gradient]
 
  //! [Source function]
  static double sourceFunction(const double x, const double y, const double z) {
    return -exp(-100. * (sqr(x) + sqr(y))) *
           (400. * M_PI *
                (x * cos(M_PI * y) * sin(M_PI * x) +
                 y * cos(M_PI * x) * sin(M_PI * y)) +
            2. * (20000. * (sqr(x) + sqr(y)) - 200. - sqr(M_PI)) *
                cos(M_PI * x) * cos(M_PI * y));
  }
  //! [Source function]
 
  static MoFEMErrorCode getTagHandle(MoFEM::Interface &m_field,
                                     const char *name, DataType type,
                                     Tag &tag_handle) {
    MoFEMFunctionBegin;
    int int_val = 0;
    double double_val = 0;
    switch (type) {
    case MB_TYPE_INTEGER:
      CHKERR m_field.get_moab().tag_get_handle(
          name, 1, type, tag_handle, MB_TAG_CREAT | MB_TAG_SPARSE, &int_val);
      break;
    case MB_TYPE_DOUBLE:
      CHKERR m_field.get_moab().tag_get_handle(
          name, 1, type, tag_handle, MB_TAG_CREAT | MB_TAG_SPARSE, &double_val);
      break;
    default:
      SETERRQ1(PETSC_COMM_SELF, MOFEM_DATA_INCONSISTENCY,
               "Wrong data type %d for tag", type);
    }
    MoFEMFunctionReturn(0);
  }
 
  MoFEMErrorCode readMesh();
  MoFEMErrorCode setupProblem();
  MoFEMErrorCode setIntegrationRules();
  MoFEMErrorCode createCommonData();
  MoFEMErrorCode assembleSystem();
  MoFEMErrorCode solveSystem();
  MoFEMErrorCode checkError(int iter_num = 0);
  MoFEMErrorCode refineOrder(int iter_num = 0);
  MoFEMErrorCode solveRefineLoop();
  MoFEMErrorCode outputResults(int iter_num = 0);
 
  struct CommonData {
    boost::shared_ptr<VectorDouble> approxVals;
    boost::shared_ptr<MatrixDouble> approxValsGrad;
    boost::shared_ptr<MatrixDouble> approxFlux;
    SmartPetscObj<Vec> petscVec;
 
    double maxErrorIndicator;
 
    enum VecElements {
      ERROR_L2_NORM = 0,
      ERROR_H1_SEMINORM,
      ERROR_INDICATOR_TOTAL,
      LAST_ELEMENT
    };
  };
 
  boost::shared_ptr<CommonData> commonDataPtr;
  struct OpError : public DomainEleOp {
    boost::shared_ptr<CommonData> commonDataPtr;
    MoFEM::Interface &mField;
    OpError(boost::shared_ptr<CommonData> &common_data_ptr,
            MoFEM::Interface &m_field)
        : DomainEleOp("U", OPROW), commonDataPtr(common_data_ptr),
          mField(m_field) {
      std::fill(&doEntities[MBVERTEX], &doEntities[MBMAXTYPE], false);
      doEntities[MBTRI] = doEntities[MBQUAD] = true;
    }
    MoFEMErrorCode doWork(int side, EntityType type, EntData &data);
  };
};
 
//! [Run programme]
MoFEMErrorCode MixedPoisson::runProblem() {
  MoFEMFunctionBegin;
  CHKERR readMesh();
  CHKERR setupProblem();
  CHKERR createCommonData();
  CHKERR solveRefineLoop();
  MoFEMFunctionReturn(0);
}
//! [Run programme]
 
//! [Read mesh]
MoFEMErrorCode MixedPoisson::readMesh() {
  MoFEMFunctionBegin;
  CHKERR mField.getInterface(simpleInterface);
  CHKERR simpleInterface->getOptions();
  CHKERR simpleInterface->loadFile();
  MoFEMFunctionReturn(0);
}
//! [Read mesh]
 
//! [Set up problem]
MoFEMErrorCode MixedPoisson::setupProblem() {
  MoFEMFunctionBegin;
 
  CHKERR simpleInterface->addDomainField("U", L2, AINSWORTH_LEGENDRE_BASE, 1);
 
  int nb_quads = 0;
  CHKERR mField.get_moab().get_number_entities_by_type(0, MBQUAD, nb_quads);
  auto base = AINSWORTH_LEGENDRE_BASE;
  if (nb_quads) {
    // AINSWORTH_LEGENDRE_BASE is not implemented for HDIV/HCURL space on quads
    base = DEMKOWICZ_JACOBI_BASE;
  }
 
  // Note that in 2D case HDIV and HCURL spaces are isomorphic, and therefore
  // only base for HCURL has been implemented in 2D. Base vectors for HDIV space
  // are be obtained after rotation of HCURL base vectors by a right angle
  CHKERR simpleInterface->addDomainField("Q", HCURL, base, 1);
 
  thetaParam = 0.5;
  CHKERR PetscOptionsGetReal(PETSC_NULL, "", "-theta", &thetaParam, PETSC_NULL);
 
  indicTolerance = 1e-3;
  CHKERR PetscOptionsGetReal(PETSC_NULL, "", "-indic_tol", &indicTolerance,
                             PETSC_NULL);
 
  initOrder = 2;
  CHKERR PetscOptionsGetInt(PETSC_NULL, "", "-order", &initOrder, PETSC_NULL);
  CHKERR simpleInterface->setFieldOrder("U", initOrder);
  CHKERR simpleInterface->setFieldOrder("Q", initOrder + 1);
  CHKERR simpleInterface->setUp();
 
  CHKERR mField.get_moab().get_entities_by_dimension(0, 2, domainEntities);
  Tag th_order;
  CHKERR getTagHandle(mField, "ORDER", MB_TYPE_INTEGER, th_order);
  for (auto ent : domainEntities) {
    CHKERR mField.get_moab().tag_set_data(th_order, &ent, 1, &initOrder);
  }
  MoFEMFunctionReturn(0);
}
//! [Set up problem]
 
//! [Set integration rule]
MoFEMErrorCode MixedPoisson::setIntegrationRules() {
  MoFEMFunctionBegin;
 
  auto rule = [](int, int, int p) -> int { return 2 * p; };
 
  PipelineManager *pipeline_mng = mField.getInterface<PipelineManager>();
  CHKERR pipeline_mng->setDomainLhsIntegrationRule(rule);
  CHKERR pipeline_mng->setDomainRhsIntegrationRule(rule);
 
  MoFEMFunctionReturn(0);
}
//! [Set integration rule]
 
//! [Create common data]
MoFEMErrorCode MixedPoisson::createCommonData() {
  MoFEMFunctionBegin;
  commonDataPtr = boost::make_shared<CommonData>();
  PetscInt ghosts[3] = {0, 1, 2};
  if (!mField.get_comm_rank())
    commonDataPtr->petscVec =
        createGhostVector(mField.get_comm(), 3, 3, 0, ghosts);
  else
    commonDataPtr->petscVec =
        createGhostVector(mField.get_comm(), 0, 3, 3, ghosts);
  commonDataPtr->approxVals = boost::make_shared<VectorDouble>();
  commonDataPtr->approxValsGrad = boost::make_shared<MatrixDouble>();
  commonDataPtr->approxFlux = boost::make_shared<MatrixDouble>();
  MoFEMFunctionReturn(0);
}
//! [Create common data]
 
//! [Assemble system]
MoFEMErrorCode MixedPoisson::assembleSystem() {
  MoFEMFunctionBegin;
  PipelineManager *pipeline_mng = mField.getInterface<PipelineManager>();
  pipeline_mng->getDomainLhsFE().reset();
  pipeline_mng->getDomainRhsFE().reset();
  pipeline_mng->getOpDomainRhsPipeline().clear();
  pipeline_mng->getOpDomainLhsPipeline().clear();
 
  CHKERR AddHOOps<2, 2, 2>::add(pipeline_mng->getOpDomainLhsPipeline(), {HDIV});
 
  auto beta = [](const double, const double, const double) { return 1; };
  pipeline_mng->getOpDomainLhsPipeline().push_back(
      new OpHdivHdiv("Q", "Q", beta));
  auto unity = []() { return 1; };
  pipeline_mng->getOpDomainLhsPipeline().push_back(
      new OpHdivU("Q", "U", unity, true));
  auto source = [&](const double x, const double y, const double z) {
    return -sourceFunction(x, y, z);
  };
  pipeline_mng->getOpDomainRhsPipeline().push_back(
      new OpDomainSource("U", source));
  MoFEMFunctionReturn(0);
}
//! [Assemble system]
 
//! [Solve]
MoFEMErrorCode MixedPoisson::solveSystem() {
  MoFEMFunctionBegin;
  PipelineManager *pipeline_mng = mField.getInterface<PipelineManager>();
  auto solver = pipeline_mng->createKSP();
  CHKERR KSPSetFromOptions(solver);
  CHKERR KSPSetUp(solver);
 
  auto dm = simpleInterface->getDM();
  auto D = createDMVector(dm);
  auto F = vectorDuplicate(D);
  CHKERR VecZeroEntries(D);
 
  CHKERR KSPSolve(solver, F, D);
  CHKERR VecGhostUpdateBegin(D, INSERT_VALUES, SCATTER_FORWARD);
  CHKERR VecGhostUpdateEnd(D, INSERT_VALUES, SCATTER_FORWARD);
  CHKERR DMoFEMMeshToLocalVector(dm, D, INSERT_VALUES, SCATTER_REVERSE);
  MoFEMFunctionReturn(0);
}
//! [Solve]
 
//! [Refine]
MoFEMErrorCode MixedPoisson::refineOrder(int iter_num) {
  MoFEMFunctionBegin;
  Tag th_error_ind, th_order;
  CHKERR getTagHandle(mField, "ERROR_INDICATOR", MB_TYPE_DOUBLE, th_error_ind);
  CHKERR getTagHandle(mField, "ORDER", MB_TYPE_INTEGER, th_order);
 
  std::vector<Range> refinement_levels;
  refinement_levels.resize(iter_num + 1);
  for (auto ent : domainEntities) {
    double err_indic = 0;
    CHKERR mField.get_moab().tag_get_data(th_error_ind, &ent, 1, &err_indic);
 
    int order, new_order;
    CHKERR mField.get_moab().tag_get_data(th_order, &ent, 1, &order);
    new_order = order + 1;
    Range refined_ents;
    if (err_indic > thetaParam * maxErrorIndicator) {
      refined_ents.insert(ent);
      Range adj;
      CHKERR mField.get_moab().get_adjacencies(&ent, 1, 1, false, adj,
                                               moab::Interface::UNION);
      refined_ents.merge(adj);
      refinement_levels[new_order - initOrder].merge(refined_ents);
      CHKERR mField.get_moab().tag_set_data(th_order, &ent, 1, &new_order);
    }
  }
 
  for (int ll = 1; ll < refinement_levels.size(); ll++) {
    CHKERR mField.getInterface<CommInterface>()->synchroniseEntities(
        refinement_levels[ll]);
 
    if (initOrder + ll > 8) {
      MOFEM_LOG("EXAMPLE", Sev::warning)
          << "setting approximation order higher than 8 is not currently "
             "supported"
          << endl;
    } else {
      CHKERR mField.set_field_order(refinement_levels[ll], "U", initOrder + ll);
      CHKERR mField.set_field_order(refinement_levels[ll], "Q",
                                    initOrder + ll + 1);
    }
  }
 
  CHKERR mField.getInterface<CommInterface>()->synchroniseFieldEntities("Q");
  CHKERR mField.getInterface<CommInterface>()->synchroniseFieldEntities("U");
  CHKERR mField.build_fields();
  CHKERR mField.build_finite_elements();
  CHKERR mField.build_adjacencies(simpleInterface->getBitRefLevel());
  mField.getInterface<ProblemsManager>()->buildProblemFromFields = PETSC_TRUE;
  CHKERR DMSetUp_MoFEM(simpleInterface->getDM());
  MoFEMFunctionReturn(0);
}
//! [Refine]
 
//! [Solve and refine loop]
MoFEMErrorCode MixedPoisson::solveRefineLoop() {
  MoFEMFunctionBegin;
  CHKERR assembleSystem();
  CHKERR setIntegrationRules();
  CHKERR solveSystem();
  CHKERR checkError();
  CHKERR outputResults();
 
  int iter_num = 1;
  while (fabs(indicTolerance) > DBL_EPSILON &&
         totErrorIndicator > indicTolerance) {
    MOFEM_LOG("EXAMPLE", Sev::inform) << "Refinement iteration " << iter_num;
 
    CHKERR refineOrder(iter_num);
    CHKERR assembleSystem();
    CHKERR setIntegrationRules();
    CHKERR solveSystem();
    CHKERR checkError(iter_num);
    CHKERR outputResults(iter_num);
 
    iter_num++;
    if (iter_num > 100)
      SETERRQ(PETSC_COMM_SELF, MOFEM_OPERATION_UNSUCCESSFUL,
              "Too many refinement iterations");
  }
  MoFEMFunctionReturn(0);
}
//! [Solve and refine loop]
 
//! [Check error]
MoFEMErrorCode MixedPoisson::checkError(int iter_num) {
  MoFEMFunctionBegin;
  PipelineManager *pipeline_mng = mField.getInterface<PipelineManager>();
  pipeline_mng->getDomainLhsFE().reset();
  pipeline_mng->getDomainRhsFE().reset();
  pipeline_mng->getOpDomainRhsPipeline().clear();
 
  CHKERR AddHOOps<2, 2, 2>::add(pipeline_mng->getOpDomainRhsPipeline(),
                                {HDIV, L2});
 
  pipeline_mng->getOpDomainRhsPipeline().push_back(
      new OpCalculateScalarFieldValues("U", commonDataPtr->approxVals));
  pipeline_mng->getOpDomainRhsPipeline().push_back(
      new OpCalculateScalarFieldGradient<2>("U",
                                            commonDataPtr->approxValsGrad));
  pipeline_mng->getOpDomainRhsPipeline().push_back(
      new OpCalculateHVecVectorField<3>("Q", commonDataPtr->approxFlux));
  pipeline_mng->getOpDomainRhsPipeline().push_back(
      new OpError(commonDataPtr, mField));
 
  commonDataPtr->maxErrorIndicator = 0;
  CHKERR VecZeroEntries(commonDataPtr->petscVec);
  CHKERR pipeline_mng->loopFiniteElements();
  CHKERR VecAssemblyBegin(commonDataPtr->petscVec);
  CHKERR VecAssemblyEnd(commonDataPtr->petscVec);
  CHKERR VecGhostUpdateBegin(commonDataPtr->petscVec, INSERT_VALUES,
                             SCATTER_FORWARD);
  CHKERR VecGhostUpdateEnd(commonDataPtr->petscVec, INSERT_VALUES,
                           SCATTER_FORWARD);
 
  MPI_Allreduce(&commonDataPtr->maxErrorIndicator, &maxErrorIndicator, 1,
                MPI_DOUBLE, MPI_MAX, mField.get_comm());
 
  const double *array;
  CHKERR VecGetArrayRead(commonDataPtr->petscVec, &array);
  MOFEM_LOG("EXAMPLE", Sev::inform)
      << "Global error indicator (max): " << commonDataPtr->maxErrorIndicator;
  MOFEM_LOG("EXAMPLE", Sev::inform)
      << "Global error indicator (total): "
      << std::sqrt(array[CommonData::ERROR_INDICATOR_TOTAL]);
  MOFEM_LOG("EXAMPLE", Sev::inform)
      << "Global error L2 norm: "
      << std::sqrt(array[CommonData::ERROR_L2_NORM]);
  MOFEM_LOG("EXAMPLE", Sev::inform)
      << "Global error H1 seminorm: "
      << std::sqrt(array[CommonData::ERROR_H1_SEMINORM]);
 
  totErrorIndicator = std::sqrt(array[CommonData::ERROR_INDICATOR_TOTAL]);
  CHKERR VecRestoreArrayRead(commonDataPtr->petscVec, &array);
 
  std::vector<Tag> tag_handles;
  tag_handles.resize(4);
  CHKERR getTagHandle(mField, "ERROR_L2_NORM", MB_TYPE_DOUBLE, tag_handles[0]);
  CHKERR getTagHandle(mField, "ERROR_H1_SEMINORM", MB_TYPE_DOUBLE,
                      tag_handles[1]);
  CHKERR getTagHandle(mField, "ERROR_INDICATOR", MB_TYPE_DOUBLE,
                      tag_handles[2]);
  CHKERR getTagHandle(mField, "ORDER", MB_TYPE_INTEGER, tag_handles[3]);
 
  ParallelComm *pcomm =
      ParallelComm::get_pcomm(&mField.get_moab(), MYPCOMM_INDEX);
  if (pcomm == NULL)
    SETERRQ(PETSC_COMM_WORLD, MOFEM_DATA_INCONSISTENCY, "Communicator not set");
 
  tag_handles.push_back(pcomm->part_tag());
  std::ostringstream strm;
  strm << "error_" << iter_num << ".h5m";
  CHKERR mField.get_moab().write_file(strm.str().c_str(), "MOAB",
                                      "PARALLEL=WRITE_PART", 0, 0,
                                      tag_handles.data(), tag_handles.size());
  MoFEMFunctionReturn(0);
}
//! [Check error]
 
//! [Output results]
MoFEMErrorCode MixedPoisson::outputResults(int iter_num) {
  MoFEMFunctionBegin;
  PipelineManager *pipeline_mng = mField.getInterface<PipelineManager>();
  pipeline_mng->getDomainLhsFE().reset();
 
  using PostProcEle = PostProcBrokenMeshInMoab<DomainEle>;
 
  auto post_proc_fe = boost::make_shared<PostProcEle>(mField);
 
  CHKERR AddHOOps<2, 2, 2>::add(post_proc_fe->getOpPtrVector(), {HDIV});
 
  auto u_ptr = boost::make_shared<VectorDouble>();
  auto flux_ptr = boost::make_shared<MatrixDouble>();
  post_proc_fe->getOpPtrVector().push_back(
      new OpCalculateScalarFieldValues("U", u_ptr));
  post_proc_fe->getOpPtrVector().push_back(
      new OpCalculateHVecVectorField<3>("Q", flux_ptr));
 
  using OpPPMap = OpPostProcMapInMoab<3, 3>;
 
  post_proc_fe->getOpPtrVector().push_back(
 
      new OpPPMap(post_proc_fe->getPostProcMesh(),
                  post_proc_fe->getMapGaussPts(),
 
                  OpPPMap::DataMapVec{{"U", u_ptr}},
 
                  OpPPMap::DataMapMat{{"Q", flux_ptr}},
 
                  OpPPMap::DataMapMat{},
 
                  OpPPMap::DataMapMat{}
 
                  )
 
  );
 
  pipeline_mng->getDomainRhsFE() = post_proc_fe;
  CHKERR pipeline_mng->loopFiniteElements();
 
  std::ostringstream strm;
  strm << "out_" << iter_num << ".h5m";
  CHKERR post_proc_fe->writeFile(strm.str().c_str());
  MoFEMFunctionReturn(0);
}
//! [Output results]
 
//! [OpError]
MoFEMErrorCode MixedPoisson::OpError::doWork(int side, EntityType type,
                                             EntData &data) {
  MoFEMFunctionBegin;
  const int nb_integration_pts = getGaussPts().size2();
  const double area = getMeasure();
  auto t_w = getFTensor0IntegrationWeight();
  auto t_val = getFTensor0FromVec(*(commonDataPtr->approxVals));
  auto t_val_grad = getFTensor1FromMat<2>(*(commonDataPtr->approxValsGrad));
  auto t_flux = getFTensor1FromMat<3>(*(commonDataPtr->approxFlux));
  auto t_coords = getFTensor1CoordsAtGaussPts();
  FTensor::Tensor1<double, 2> t_diff;
  FTensor::Index<'i', 2> i;
 
  double error_l2 = 0;
  double error_h1 = 0;
  double error_ind = 0;
  for (int gg = 0; gg != nb_integration_pts; ++gg) {
    const double alpha = t_w * area;
 
    double diff = t_val - MixedPoisson::analyticalFunction(
                              t_coords(0), t_coords(1), t_coords(2));
    error_l2 += alpha * sqr(diff);
 
    VectorDouble vec = MixedPoisson::analyticalFunctionGrad(
        t_coords(0), t_coords(1), t_coords(2));
    auto t_fun_grad = getFTensor1FromArray<2, 2>(vec);
    t_diff(i) = t_val_grad(i) - t_fun_grad(i);
    error_h1 += alpha * t_diff(i) * t_diff(i);
 
    t_diff(i) = t_val_grad(i) - t_flux(i);
    error_ind += alpha * t_diff(i) * t_diff(i);
 
    ++t_w;
    ++t_val;
    ++t_val_grad;
    ++t_flux;
    ++t_coords;
  }
 
  const EntityHandle ent = getFEEntityHandle();
  Tag th_error_l2, th_error_h1, th_error_ind;
  CHKERR MixedPoisson::getTagHandle(mField, "ERROR_L2_NORM", MB_TYPE_DOUBLE,
                                    th_error_l2);
  CHKERR MixedPoisson::getTagHandle(mField, "ERROR_H1_SEMINORM", MB_TYPE_DOUBLE,
                                    th_error_h1);
  CHKERR MixedPoisson::getTagHandle(mField, "ERROR_INDICATOR", MB_TYPE_DOUBLE,
                                    th_error_ind);
 
  double error_l2_norm = sqrt(error_l2);
  double error_h1_seminorm = sqrt(error_h1);
  double error_ind_local = sqrt(error_ind);
  CHKERR mField.get_moab().tag_set_data(th_error_l2, &ent, 1, &error_l2_norm);
  CHKERR mField.get_moab().tag_set_data(th_error_h1, &ent, 1,
                                        &error_h1_seminorm);
  CHKERR mField.get_moab().tag_set_data(th_error_ind, &ent, 1,
                                        &error_ind_local);
 
  if (error_ind_local > commonDataPtr->maxErrorIndicator)
    commonDataPtr->maxErrorIndicator = error_ind_local;
 
  int index = CommonData::ERROR_L2_NORM;
  constexpr std::array<int, CommonData::LAST_ELEMENT> indices = {
      CommonData::ERROR_L2_NORM, CommonData::ERROR_H1_SEMINORM,
      CommonData::ERROR_INDICATOR_TOTAL};
  std::array<double, CommonData::LAST_ELEMENT> values;
  values[0] = error_l2;
  values[1] = error_h1;
  values[2] = error_ind;
  CHKERR VecSetValues(commonDataPtr->petscVec, CommonData::LAST_ELEMENT,
                      indices.data(), values.data(), ADD_VALUES);
  MoFEMFunctionReturn(0);
}
//! [OpError]
 
int main(int argc, char *argv[]) {
  // Initialisation of MoFEM/PETSc and MOAB data structures
  const char param_file[] = "param_file.petsc";
  MoFEM::Core::Initialize(&argc, &argv, param_file, help);
 
  // Add logging channel for example problem
  auto core_log = logging::core::get();
  core_log->add_sink(
      LogManager::createSink(LogManager::getStrmWorld(), "EXAMPLE"));
  LogManager::setLog("EXAMPLE");
  MOFEM_LOG_TAG("EXAMPLE", "MixedPoisson");
 
  try {
    //! [Register MoFEM discrete manager in PETSc]
    DMType dm_name = "DMMOFEM";
    CHKERR DMRegister_MoFEM(dm_name);
    //! [Register MoFEM discrete manager in PETSc
 
    //! [Create MoAB]
    moab::Core mb_instance;              ///< mesh database
    moab::Interface &moab = mb_instance; ///< mesh database interface
    //! [Create MoAB]
 
    //! [Create MoFEM]
    MoFEM::Core core(moab);           ///< finite element database
    MoFEM::Interface &m_field = core; ///< finite element database interface
    //! [Create MoFEM]
 
    //! [MixedPoisson]
    MixedPoisson ex(m_field);
    CHKERR ex.runProblem();
    //! [MixedPoisson]
  }
  CATCH_ERRORS;
 
  CHKERR MoFEM::Core::Finalize();
}