Chemical Kinetics For Exoplanetary Atmospheres
Switch branches/tags
Clone or download


Chemical kinetics for exoplanetary atmospheres, a fast and easy-to-use python code.

The theory paper of VULCAN can be found at

This is currectly a working, beta version. Any questions or feedbacks are welcome and can be sent to Shami Tsai


VULCAN is developed with Python 2.7 and does not work with Python 3. A python 3.X compatible branch will be released later. For the moment, if you are only using Python 3, it is easiest to create a separate Python 2 environment with an environment manager, e.g. Anaconda.

VULCAN requires the following python packages:

  • numpy
  • scipy
  • Sympy
  • matplotlib
  • PIL/Pillow (optional)

If any of the first four packages are not installed, the easiest way is to install the full SciPy Stack. e.g.

python -m pip install --upgrade pip
pip install --user numpy scipy matplotlib ipython jupyter pandas sympy nose

The above commands update pip and install SciPy via pip. Further information can be found at

PIL or Pillow is a plotting library. If installed, the plots will be conveniently shown by the os-built-in image viewer. See for more information.

Quick Start

Let's dive in and see how disequilibrium chemistry works!

Go to the main folder of VULCAN, run


You should see the model for HD 189733b starts running with real-time plotting. It would take a few minutes. Now a perfect time for a coffee break and watch the chemistry play!

After the run successfully finished, we can plot the results from the output (stored in /output by default). Run the plotting script, followed by three arguments: {output path} {comma-separated species} {plotname}. For example,

python output/test.vul CH4,CO,CO2,H2O,C2H2 hd189

CH4,CO,CO2,H2O,C2H2 are chosen to be plotted from test.vul and saved as hd189.eps in the /plot folder.

Now you may want to try different T-P inputs or elemental abundances. All these settings are wrapped in For example, find and edit

const_Kzz = 1.E6


C_H = 6.0618E-4

for a smaller diffusion (Kzz) and C/O carbon rich run. Set use_live_plot = False if you wish to switch off the real-time plotting. More detailed instruction can be found in the following sections. Have fun!

Full instruction


│   ├── atm/
│   ├── /thermo/
│   │   ├── HOC_compose.txt
│   │   ├── gibbs_text.txt
│   │   ├──/NASA9/
│   ├──  
│   ├── chem_funs
│   ├── CHO_netowrk.txt
│   ├──
│   ├──
│   ├──
│   ├──
│   ├──
│   ├──
│   ├──

/atm/ : for input atmospheric files
/thermo/ : for thermodynamic data
/thermo/HOC_compose : basic physical property e.g. number of atoms and molecular weight
/NASA9/ : to store NASA-9 polynomials for the Gibbs free energy of every species : modules to construct the atmospheric structure and to read in reaction rates : the functions of chemical sources, Jacobian matrix and the equilibrium constants
CHO_netowrk.txt : the default C-H-O kinetics network : modules for the main computation : pre-pipeline routine to produce : read-in and plotting script : physical constants : modules to store all the variables : the main file of VULCAN : the configuration file for VULCAN includes all the settings and parameters, e.g. the atmospheric parameters, the elemental abundance etc. Typically this is the only file you need to edit for each specific run. A summary of every setting is listed in vulcan_cfg_readme.txt.

Input Files

The key input files of VULCAN include the chemical network and the atmospheric file. CHO_network.txt is the deafult reaction network including carbon, hydrogen, and oxygen species. It is constructed for thermochemistry from 500 to 2500 K using a reduced network with 29 gaseous species and less than 300 reactions to benefit for efficiency. The rate coefficients A, B, C are written in A, B, C as in the Arrhenius formula k = A T^B exp(-C/T) (See the section of editing or using a different chemical network.) The input temperature-pressure profiles are placed in the /atm folder by default. As the form of the included example files for HD 189733b and HD 209458b, the first line is always commented for units, and the second line specifies the column names: Pressure Temp or Pressure Temp Kzz (if Kzz-P profile is provided). So the file consists of two columns without Kzz and three columns with Kzz. When Kzz is provided, set

Kzz_prof = 'file'

in the configuration file


VULCAN is developed in a flexible way that the chemical network is not hard coded. Instead, generates all the required funtions from CHO_netowrk.txt into . is executed prior to the release to make from the default network. Once any change is made to the file of chemical network, has to be executed again before running the main code to regenerate the corresponding functions, by simply run


One can examine the information, e.g. the reaction table in the comments in, being updated.

Editing or Using a different chemical network

You can edit the default netowrk, to remove or add reactions, to change rate constats, etc. You can also use a different chemical network, as long as it is in the same format as CHO_netowrk.txt. Noted that however, changing or using a different chemical network is not warranted, unrealistic values could lead to numerical issues. In the network file, the reactions should be writen in the form of [ A + B -> C + D ], including the square brackets. Only the forward reactions are listed, since VULCAN reverses the forward reactions to obtain the reverse reactions using the thermodynamic data. The reaction number, i.e. id, is irrelevent as it will be automatically generated (and writing into the network file) while running Three-body or dissociation reactions should be separately listed after the comment line

# 3-body and Disscoiation Reactions

Next, make sure all the species are included in the NASA9 folder. If not, they need to be add manually by looking over nasa9_2002_E.txt and save them into txt files with the same name as used in the network. The format of the NASA 9 polynomials is as follows

a1 a2 a3 a4 a5
a6 a7 0. a8 a9

Noted that a7 and a8 are separated by 0. The first two rows are for low temperature (200 - 1000 K) and the last two rows are for high temperature (1000 - 6000 K).

Finally, run as described before.


VULCAN is distributed under the terms of the GNU General Public License (GPL) license. For more information, see GPL_license.txt in the main directory.


The TEA compendium to reproduce Figure 2. in Tsai et al. (2017) can be found here.

The project is financially support from the Center for Space and Habitability (CSH), the PlanetS NCCR framework and the Swiss- based MERAC Foundation. The Exoclime Simulation Platform (ESP) develops a set of open-source codes for research on exoplanets. The three parts of the ESP are

  • HELIOS radiative transfer and retrieval,
  • THOR atmospheric fluid dynamics,
  • VULCAN atmospheric chemistry.