if(divSigmaExpMethod == "standard")
{
    //- calculating the full gradient has good convergence and no high freq oscillations
    divSigmaExp = fvc::div(C && epsilon) - fvc::div(K & gradU);
}
else if(divSigmaExpMethod == "surface")
{
    //- this form seems to have the best convergence
    divSigmaExp = fvc::div
    (
        mesh.magSf()*
        (
            (n & (Cf && fvc::interpolate(symm(gradU))))
          - (n & (Kf & fvc::interpolate(gradU)))
        )
    );
}
else if(divSigmaExpMethod == "laplacian")
{
    //- causes high freq oscillations and slow convergence
    divSigmaExp = fvc::div(sigma) - fvc::laplacian(K, U);
}
else
{
    FatalErrorIn(args.executable())
        << "divSigmaExp method " << divSigmaExpMethod << " not found!" << endl;
}