In MNDO and its descendants (Sewart's PMx models, Austin's AM1, MNDO/d, etc.), the nonvanishing two-center two-electron "coulomb integrals" are evaluated via a classical multipole expansion in a local coordinate system, and then transformed into global coordinates via "standard rotational transformations". How exactly are these transformations carried out?

A similar question would be the methodology behind the resonance integrals in the orthogonalised methods OMx, which are similarly evaluated in a diatomic coordinate system and then transformed to the global coordinate system.

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    $\begingroup$ There's a nice open-source C++ implementation of NDDO methods in Sparrow - you can probably hunt around in the code and/or ask questions of the Reiher team. $\endgroup$ Commented Apr 29, 2020 at 23:52
  • $\begingroup$ @GeoffHutchison Does anyone know where to get MNDO99? Walter Thiel was the last known developer but has since passed. I'm interested in using it for OM2/MRCI calculations. If you do, let me know and I'll reward you with a bounty on materials SE (50 pts)! $\endgroup$
    – Cody Aldaz
    Commented May 4, 2020 at 22:12
  • $\begingroup$ @CodyAldaz - A Google search turned up a stub of a project on GitHub: github.com/andersx/pymndo99 - I'd contact Anders and ask him. Martin Korth and Paolo Drahl both worked with Walter and may have copies. I'd definitely be curious to benchmark OMx methods.. $\endgroup$ Commented May 5, 2020 at 0:09
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    $\begingroup$ @Cody Aldaz I believe you’re looking for MNDO2005 7.0 (released 2015); I’m looking for it myself. I think your best bet is to contact Thiel’s secretariat (see the Max-Planck-Institüt website) $\endgroup$ Commented May 6, 2020 at 1:13
  • $\begingroup$ MNDO2020 (MNDO99) from Prof. Walter Thiel is now free and open source under a special MPI license. Contact: kofo.mpg.de/en/research/molecular-theory-and-spectroscopy $\endgroup$
    – Tobi
    Commented May 20, 2021 at 0:13