i get it now .. sum() is a density sum, not the number of electrons ... i missed that part ..

So then i think this is correct:

s1 = diffDensity1[:, :, :].sum()

s2 = diffDensity2[:, :, :].sum()

dX1, dY1, dZ1 = diffDensity1.volumeElement()

dX2, dY2, dZ2 = diffDensity2.volumeElement()

s1 = numpy.dot(dZ1,numpy.cross(dX1,dY1))*s1.inUnitsOf(Bohr**-3)

s2 = numpy.dot(dZ2,numpy.cross(dX2,dY2))*s2.inUnitsOf(Bohr**-3)

print "Total # electrons: ", s1, s2

numElecDiff = (s2-s1)

print "numElecDiff", numElecDiff

Where the end answer is the number of electrons added from the increase in voltage, since densities 1 and 2 were at lower and higher voltages, respectively .. but something is not right here, because the # of electrons is less than 1 over the entire volume ... or is that per unit volume? if per unit volume, how do i get the number of electrons ... ?