HFSS finding capacitance of MEMS

I am trying to find the capacitance of a MEMS device. However the problem with that, that I dont know where exactly to assign the ground. I know how to assign the radiation boundary, and i have 2 lumped ports. When i simulate without ground, i get a Z matrix that is not really what im expecting, (farads), My Z0 (for lumped port) is 50 ohm, but i dont know how does that affect the Z matrix.

So, Where to assign the ground boundary given symmetric MEMS?
How to calculate C if your Z0 of lumped port is 50 ohm?

Thank you for helping :D

Would you mind to sketch the topology, how you connect the capacitance to the ports and where you have a ground?

Also, is the frequency range so that EM effects have to be considered or is it essentially an AC capacitance problem?

this is the memes, i want to find the capacitance from the two ports, left and right

This is it im trying to find the capacitance across its length

I dont have a ground, i just want to find the discrete capacitance of that model.

Frequency range is from 20k hz to 1Mhz for example
It is an AC capacitance really to find the discrete C.

If the topology is like below, ground definitely matters, particularly the ground capacitance of the center part.

Or is it that the center part has also an electrical connection?

Yeah the purpose of this topology is to maximize capacitance. So in order to simulate this correctly, how should i place the ground? the center part has no electrical connection