Coplanar Grid Detector Capacitance

Hi all,

I'm fairly new to Comsol and I'm trying to obtain the capacitance for a CdZnTe coplanar grid detector (CPG).
I've read up on how the the parallel plate capacitor is built up and tried it out for myself.

I've obtained a capacitance from the CPG model that I've attached, but I'm confused by how changing the terminals affects the result. I've set the underside of the detector as the only terminal, whislt grounding the andoe (collecting grid) and set a negative potential on the non-collecting grid. From this I evaluate a capacitence of 0.879pF.

If I run the model again but set the non-collecting grid to ground, I get a different capacitence of 0.907pF. I'm a bit confused as I was thinking that altering potentials shouldn't effect the capacitance?

Am I using these terminals correctly? Should these capacitances change?

Many thanks,

Richard.

Note: I set the mesh very high for testing, it might not need to be this fine.