@User: The ".sim" file is only needed because someone a long time ago made a choice to use the .sim format to read back information about nets in the extracted layout to use to compute the parasitic resistances. The problem is that the .sim file format is very, very simple and is missing some of the information that would be needed for a proper parasitic extraction, like where the substrate is, and how to handle devices other than MOSFETs. I have added in code that deals with both to some extent, but much of the code needs to be rewritten to stop depending on the .sim format and just using the extraction result from the .ext files instead.