Like in GEOS-Chem Classic, the HEMCO_Diagn.rc file is used to map between HEMCO containers and output file diagnostic names. However, while all uncommented diagnostics listed in HEMCO_Diagn.rc are output as HEMCO diagnostics in GEOS-Chem Classic, only the subset also listed in HISTORY.rc are output in GCHP. See the HEMCO documentation for an overview of the file.