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The GEOS-Chem interface to CESM is in directory src/chemistry/geoschem, primarily in file chemistry.F90. In the GEOS-Chem compsets, it entirely replaces overloads subroutines chem_gasphase_chemdr and chemistry_timestep_tend used in CAM-chem. The actual externally maintained GEOS-Chem source code is a modified copy in in src/chemistry/geoschem/geoschem_src.
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