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

my results for reference:

`2.49328980937 1/Bohr**3 2.62952644165 1/Bohr**3 3.18669569532 1/Bohr**3 0.136236632277 1/Bohr**3 0.557169253673 1/Bohr**3`

unit lengths [ 0.69774531 0. 0. ] Bohr [ 0. 0.69769726 0. ] Bohr [ 0. 0. 0.70126857] Bohr

unit lengths [ 0.69774531 0. 0. ] Bohr [ 0. 0.69769726 0. ] Bohr [ 0. 0. 0.70126857] Bohr

volumes 0.341388054851 Bohr**3 0.341388054851 Bohr**3 0.341388054851 Bohr**3

Total # electrons: 0.851179358199 0.897688917092 1.08789984483

numElecDiff 0.0465095588926 0.190210927734

capacitance 1.48830588456e-19 1.2173499375e-19

capacitance in fF 0.000148830588456 0.00012173499375