Info<< "Reading field DU\n" << endl;
    volVectorField DU
    (
        IOobject
        (
            "DU",
            runTime.timeName(),
            mesh,
            IOobject::MUST_READ,
            IOobject::AUTO_WRITE
        ),
        mesh
    );

    volTensorField gradDU = fvc::grad(DU);

    Info<< "Creating field U\n" << endl;
    volVectorField U
    (
        IOobject
        (
            "U",
            runTime.timeName(),
            mesh,
            IOobject::READ_IF_PRESENT,
            IOobject::AUTO_WRITE
        ),
        mesh,
        dimensionedVector("zero", dimLength, vector::zero)
    );

    volSymmTensorField DEpsilon
    (
        IOobject
        (
	 "DEpsilon",
	 runTime.timeName(),
	 mesh,
	 IOobject::READ_IF_PRESENT,
	 IOobject::AUTO_WRITE
	 ),
        mesh,
        dimensionedSymmTensor("zero", dimless, symmTensor::zero)
     );

    volSymmTensorField DSigma
    (
        IOobject
        (
            "DSigma",
            runTime.timeName(),
            mesh,
            IOobject::READ_IF_PRESENT,
            IOobject::AUTO_WRITE
        ),
        mesh,
        dimensionedSymmTensor("zero", dimForce/dimArea, symmTensor::zero)
    );

    volSymmTensorField sigma
    (
        IOobject
        (
            "sigma",
            runTime.timeName(),
            mesh,
            IOobject::READ_IF_PRESENT,
            IOobject::AUTO_WRITE
        ),
        mesh,
        dimensionedSymmTensor("zero", dimForce/dimArea, symmTensor::zero)
    );

    volSymmTensorField epsilon
    (
        IOobject
        (
            "epsilon",
            runTime.timeName(),
            mesh,
            IOobject::READ_IF_PRESENT,
            IOobject::AUTO_WRITE
        ),
        mesh,
        dimensionedSymmTensor("zero", dimless, symmTensor::zero)
    );

    volSymmTensorField epsilonP
    (
        IOobject
        (
            "epsilonP",
            runTime.timeName(),
            mesh,
            IOobject::READ_IF_PRESENT,
            IOobject::AUTO_WRITE
        ),
        mesh,
        dimensionedSymmTensor("zero", dimless, symmTensor::zero)
    );


      volVectorField divDSigmaExp
      (
        IOobject
        (
	 "divDSigmaExp",
	 runTime.timeName(),
	 mesh,
	 IOobject::NO_READ,
	 IOobject::NO_WRITE
	 ),
        mesh,
        dimensionedVector("zero", dimensionSet(1,-2,-2,0,0,0,0), vector::zero)
       );

    constitutiveModel rheology(sigma, DU);

    volScalarField rho = rheology.rho();
    volScalarField mu = rheology.mu();
    volScalarField lambda = rheology.lambda();
    surfaceScalarField muf = fvc::interpolate(mu, "mu");
    surfaceScalarField lambdaf = fvc::interpolate(lambda, "lambda");

    surfaceVectorField n = mesh.Sf()/mesh.magSf();


   // for aitken relaxation
   volVectorField aitkenDelta
    (
        IOobject
        (
            "aitkenDelta",
            runTime.timeName(),
            mesh,
            IOobject::NO_READ,
            IOobject::NO_WRITE
        ),
        mesh,
	dimensionedVector("zero", dimLength, vector::zero)
    );
   scalar aitkenInitialRes = 1.0;
   scalar aitkenTheta = 0.1;
   if(mesh.relax(DU.name()))
   {
     aitkenTheta = mesh.relaxationFactor(DU.name());
   }