@User Short answer: you can't. Long answer: ngspice has experimental transient noise simulation you could try and then apply some post processing. Another free (as in beer) option is to use the PAN simulator from https://brambilla.deib.polimi.it/ which has some powerful features including pnoise but some self assembly is required

@yrrapt Thank you sir. I have installed PAN simulator. I could run the example files given using diode models. I tried copying the netlist generated by xschem and expanded by ngspice for a simple inverter. I have changed few parameters to satisfy PAN simulator syntax. But i am still not able to run the simulations. I have attached the netlist. can you please let me know where I am going wrong.

@yrrapt Could you please let me know how to do transient noise simulations in ngspice? I have done my simulations using the open source PDK (Sky130nm) which has noise files included in it. From the Ngspice manual I could find example transient noise simulations with only external noise source being included. I have also gone through other Ngspice forum links related to this but couldn't understand the procedure clearly. Kindly help.