Usually you should specify e.g. %chk=/home/sob/abc.chk in your .gjf file. Then after calculation, the chk file will be generated. After that you can use formchk utility in Gaussian to convert it to fch/fchk.In addition, if you set "formchkpath" in settings.ini to actual path of formchk executable file, then Multiwfn will be able to directly open .chk file, since formchk will be automatically invoked.
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