@User (@User): My experience with ngspice is that any node that is floating will cause a singular matrix. There is no default resistance to ground that I know of. The good news is that the issue can be resolved with .option RSHUNT, and the value of RSHUNT can be ridiculously high so as not to be able to affect circuit behavior in any way, but will still resolve the singular matrix issue. e.g., you can set RSHUNT to 1e20 or similar.