Telling GEOS-Chem to use a custom mechanism

GEOS-Chem will always use the default mechanism (which is named fullchem or Standard depending on which version you have). To tell GEOS-Chem to use the custom mechanism, follow these steps.

For GEOS-Chem versions using CMake

If your GEOS-Chem version uses CMake, thenen navigate to your build directory.


GEOS-Chem Classic run directories have a subdirectory named build in which you can configure and build GEOS-Chem. If you don’t have a build directory, you can add one to your run directory.

For more information about the GEOS-Chem and GCHP configuration process, please see GEOS-Chem manual and

Configuring GEOS-Chem

From the build directory, type:

$ cmake ../CodeDir -DCUSTOMMECH=y -DRUNDIR=..

NOTE: In some later GEOS-Chem 12 versions, the proper command is:

$ cmake ../CodeDir -DCHEM=custom

Please consult the GEOS-Chem manual for more information.

You should see output similar to this written to the screen:

-- General settings:

This confirms that the custom mechanism has been selected.

Compiling GEOS-Chem

Once you have configured GEOS-Chem to use the custom mechanism, you may build the exectuable. Type:

$ make -j
$ make -j install

The executable file (gcclassic or gchp, depending on which mode of GEOS-Chem that you are using) will be placed in the run directory.

For GEOS-Chem versions using GNU Make

If you are using an older version of GEOS-Chem that is only compatible with GNU Make, then use this command to compile the executable:

$ make -j CHEM=Custom ...etc other build options...

Please consult the GEOS-Chem manual for more information.