Hi, danda821: Normally, those negative values (mainly C's) are not important. Their values are very small and they can be dropped. They are there because the fitting numerically fits them like that. You normally should simulate a structure at low frequency. Then, you do the PROCESS->LC EQUIVALENT to find the SPICE equivalent circuit. If you select FILE->SAVE SPICE FILE, you can save the equivalent circuit into SPICE format and you can use it on a SPICE simulation.