FastCap and FastHenry, from MIT (Massachusetts Institute of Technology) are two free parasitics extractor tools for capacitance, and inductance and resistance. Quoted in many scientific articles, are considered golden references in their field.
FastHenry is included as a compiled stand alone version in any LayoutEditor package and it can be used separate from the LayoutEditor by manual call. However the most convenient way to use is via the ExtractionInductance within the LayoutEditor. Any documentation and also the source code for it, you will find on this page.
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