Finding impedance between two terminals?

Hi there,

I have a simple coaxial structure with four electrodes placed in a line. I apply a voltage to the outer two terminals by:

T1: 0.5*sin(2*pi*es.freq) *(outer left)*
T2: -0.5*sin(2*pi*es.freq) *(outer right)*
(The inner two conductors are both Charge Terminal Type with 0 C and 0 V)

I'm using the ES physics. Now I'd like to find the impedance across these two inner terminals. Currently, I can only solve for V, I and Q at the terminals. I do (es.v0_3 - es.v0_4)/(es.I0_4) to find Z, but get an error because es.I0_4 is 0A.

How can I find Z?

Thanks in advance