I have seen this (negative capacitance) problem before, and most SPICE
programs do not handle it well. An exception is Micro-Cap V from Spectrum
Software in Sunnyvale, CA. You can indeed use negative capacitance values in
models created in Micro-Cap V (which is based on Berkeley SPICE 3f5).
MC5 will also directly run SPICE text files, but I haven't tried negative
capacitance in the text format (yet).
In the interim, negative capacitance is the equivalent of postive inductance.
Work out the equations yourself. Then, if your SPICE program is capable of
it, use a frequency dependent model for the capacitor that reflects the
equivalent inductance. Or perhaps a .PARAM statement could incorporate these
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