Thank you .. i will post the final script, could be helpful to others after all this discussion ...
This is a 2 probe GNR TFET device, 16nm long, 1.29nm wide

```
diffDensity1 = nlread(fName, ElectronDifferenceDensity, object_id="diffDensity[vg-0.25000][vds0.50000]")[0]
diffDensity2 = nlread(fName, ElectronDifferenceDensity, object_id="diffDensity[vg-0.20000][vds0.50000]")[0]
diffDensity3 = nlread(fName, ElectronDifferenceDensity, object_id="diffDensity[vg0.00000][vds0.50000]")[0]
s1 = diffDensity1[:, :, :].sum()
s2 = diffDensity2[:, :, :].sum()
s3 = diffDensity3[:, :, :].sum()
print s1, s2, s3, s2-s1, s3-s2
# Find the volume elements.
dX1, dY1, dZ1 = diffDensity1.volumeElement()
dX2, dY2, dZ2 = diffDensity2.volumeElement()
dX3, dY3, dZ3 = diffDensity3.volumeElement()
print "unit lengths", dX1,dY1,dZ1
print "unit lengths", dX2,dY2,dZ2
length_unit1 = dX1.unit()
length_unit2 = dX2.unit()
length_unit3 = dX3.unit()
# Calculate the volume of the volume element.
dV1 = numpy.dot(dX1, numpy.cross(dY1,dZ1)) * length_unit1**3
dV2 = numpy.dot(dX2, numpy.cross(dY2,dZ2)) * length_unit2**3
dV3 = numpy.dot(dX3, numpy.cross(dY3,dZ3)) * length_unit3**3
print "volumes", dV1, dV2, dV3
numE1 = s1*dV1
numE2 = s2*dV2
numE3 = s3*dV3
print "Total # electrons: ", numE1, numE2, numE3
# let's use s2-s1 as difference in # electrons in channel due to voltage
numElecDiff1 = (numE2-numE1)
numElecDiff2 = (numE3-numE2)
print "numElecDiff", numElecDiff1, numElecDiff2
# convert to coulombs
chargeDiff1 = numElecDiff1*1.6e-19
chargeDiff2 = numElecDiff2*1.6e-19
voltDiff1 = 0.05
voltDiff2 = 0.25
capacitance1 = chargeDiff1/voltDiff1
capacitance2 = chargeDiff2/voltDiff2
print "capacitance", (capacitance1), (capacitance2)
print "capacitance in fF", (capacitance1)*1e15, (capacitance2)*1e15
```