I'm seeing some strange ngspice behaviour. I have an uncompensated opamp with an external compensation pin. When I connect a compensation cap to the compensation pin of the amp. spice complains of a singular matrix and appears to run the simulation ignoring the cap completely, producing an AC response as though the cap were not connected. See below. Ignore the 0A current source in parallel with Cc its just to stop xschem from highlighting the nets when the cap is removed, and the simulation results are the same with or without it.