Difference between terminal Boundary condition and Electric potential boundary condition.

Hi,

I am modelling a free PZT patch.

I have to apply a voltage potential of 20V across the top and bottom of the PZT patch. (attaching the model)

I am doing it via two methods,
(a) applying "electric potential" boundry condition at top surface and specifying the bottom surface as ground.
(b) applying "terminal" boundry condition at top surface and specifying the bottom surface as ground.

Ultimate aim is to find admittance of piezo.

In the first method(a) I am doing it by A= I/V, where, V= line_integral(pzd.Ey) and I = surface_integral(pzd.Jy)
and in the second method (b) Admittance = pzd.Y11

However the value of admittance in two methods is not same.
Please suggest which is the right way of getting admittance and what is the difference between two boundary conditions of "terminal' and 'electric potential'

Thank you in advance.

Regards,
Naveet