Speed up a slow simulation

GEOS-Chem Classic performance is continuously monitored by the GEOS-Chem Support Team by means of benchmark simulations and ad-hoc timing tests. It has been shown that running GEOS-Chem with recommended timesteps from Philip et al. [2016]. can increase run times by approximately a factor of 2. To speed up GEOS-Chem Classic simulations, users may choose to use any of the following options.

Use coarser timesteps

As discussed previously, the default timesteps for GEOS-Chem Classic are 600 seconds for dynamics, and 1200 seconds for chemistry and emissions. You can experiment with using coarser timesteps (such as 1800 seconds for dynamics and 3600 seconds for emissions & chemistry).


For nested-grid simulations, you might not be able to use coarser timesteps, or else the Courant limit in transport will be violated.

Turn off unwanted diagnostics

Several diagnostics are turned on by default in the HISTORY.rc configuration file. The more diagnostics that are turned on, the more I/O operations need to be done, resulting in longer simulation execution times. Disabling diagnostics that you do not wish to archive can result in a faster simulation.

Disable debugging options

If you previously configured GEOS-Chem with the : CMAKE_BUILD_TYPE option set to Debug, then several run-time debugging checks will be activated. These include:

  • Checking for array-out-of-bounds errors

  • Checking for floating-point math exceptions (e.g. div-by-zero)

  • Disabling compiler optimizations

These options can be useful in detecting errors in your GEOS-Chem Classic simulation, but result in a much slower simulation. If you plan on running a long Classic simulation, make sure that you configure and build GEOS-Chem Classic so that CMAKE_BUILD_TYPE is set to Release.