How to run case with explicitCoupling

Hi,
I was wondering how can I run a case using explicitCoupling. I understand that implicit coupling provides better stability, but based on what I've been reading in the literature the explicit coupling is more accurate. So referring to the settling case tutorial, cfdemSolverPiso/settlingTestMPI, I made the following adjustments in constant/couplingProperties:

//--------------------------------------------------------------------
momCoupleModels
(
explicitCouple
//implicitCouple
);

// sub-model properties
explicitCoupleProps
{
}

DiFeliceDragProps
{
velFieldName "U";
voidfractionFieldName "voidfraction";
granVelFieldName "Us";
verbose true;
treatForceExplicit true; //<--- added the following option
}
//--------------------------------------------------------------------

but when I run the case, i get the following error:
//---------------------------------------------------------------------
Selecting momCoupleModel explicitCouple
[0]
[1]
[0]
[0] --> FOAM FATAL ERROR:
[0]
request for volVectorField f from objectRegistry region0 failed
available objects of type volVectorField are

9
(
IBDragPerV
divTau
U
expParticleForces
vSmoothField
UsNext
impParticleForces
UsPrev
Us
)
//------------------------------------------------------------------

which seems to be asking for a missing volfield "f" in the zero folder. how should I set up the "f" which seems to represent the explicit coupling source term. Would have checked earlier versions of CFDEM coupling tutorials which made use of the explicit method, but don't know how to access them.

Best
Ali