Some simulators, especially the microwave-centric ones, do not offer classical AC analysis. I've never used Nexxim, but I am aware of others that don't.

You can compute all the quantities you need from multi-port matrix parameters, either directly or as functions of the matrix elements.

For example, the open-circuit voltage gain of your opamp is an element of the ABCD a.k.a chain matrix, if Nexxim lets you compute it.

Except for the S-parameters, most other network descriptions are port impedance-independent, so it really doesn't matter what you specify.

Vuk