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);
    volTensorField gradDU
    (
        IOobject
        (
            "grad(DU)",
            runTime.timeName(),
            mesh,
            IOobject::NO_READ,
            IOobject::NO_WRITE
        ),
        mesh,
        dimensionedTensor("zero", dimless, tensor::zero)
    );
    surfaceVectorField snGradDU
    (
        IOobject
        (
            "snGrad(DU)",
            runTime.timeName(),
            mesh,
            IOobject::NO_READ,
            IOobject::NO_WRITE
        ),
	mesh,
	dimensionedVector("zero", dimless, vector::zero)
    );

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

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

      volVectorField divDSigmaExp
      (
        IOobject
        (
         "divDSigmaExp",
         runTime.timeName(),
         mesh,
         IOobject::NO_READ,
         IOobject::NO_WRITE
         ),
        mesh,
        dimensionedVector("zero", dimForce/dimVolume, vector::zero)
       );

constitutiveModel rheology(sigma, DU);
    //solidInterface interface(mesh, rheology);
//     solidInterface* interfacePtr = new solidInterface(mesh, rheology);

    surfaceVectorField traction
    (
        IOobject
        (
            "traction",
            runTime.timeName(),
            mesh,
            IOobject::NO_READ,
            IOobject::NO_WRITE
        ),
        mesh,
        dimensionedVector("zero", dimForce/dimArea, vector::zero)
    );

  // for aitken relaxation
   volVectorField aitkenDelta
    (
        IOobject
        (
            "aitkenDelta",
            runTime.timeName(),
            mesh,
            IOobject::NO_READ,
            IOobject::NO_WRITE
        ),
        mesh,
	dimensionedVector("zero", dimLength, vector::zero)
    );
// aitken relaxation factor
scalar aitkenInitialRes = 1.0;
scalar aitkenTheta = 0.1;