Mr. Bonanata,

Try with these keywords:

ONIOM(level of theory/level of theory:level of theory) nosymm pop=nboread iop(5/13=1) noraff scf=(maxcycle=50,converg=50)

I'll give it a try, thanks!

Dear Jenner, many thanks for asking this very interesting technical question. In this context please have a look the following useful guide:

Investigating the Reactivity and Spectra of Large Molecules with ONIOM

https://gaussian.com/oniom_technote/

In particular please see the last paragraph of this article entitled "Further Reading about ONIOM" in which additional helpful references are suggested.

Good luck with your research and best wishes, Frank Edelmann