We present an open-source, Thermochemical Equilibrium Abundances (TEA) code that calculates the molecular equilibrium abundances of the species present in planetary atmospheres. There are two methods to calculate chemical equilibrium: with kinetics by using equilibrium constants and reaction rates or by minimizing the free energy of the system. Chemical equilibrium can be calculated almost trivially for several reactions present in the system; however, as their number increases, it becomes difficult to solve the large number of equilibrium-constant relations. An advantage of the free-energy-minimization method is that each species present in the system can be treated independently. The complicated sets of reactions do not need to be specified and just a limited set of equations needs to be solved. TEA uses Gibbs-free-energy minimization calculation with an iterative Lagrangian optimization scheme based on White et al. (1958) and Eriksson (1971). The code, written in Python, is modular and documented and available to the scientific community via https://github.com/dzesmin/TEA.
TEA initializes the atmospheric retrieval calculations in the open-source Bayesian Atmospheric Radiative Transfer (BART) code. BART characterizes planetary atmospheres based on the observed spectroscopic information. It initializes a planetary atmosphere model, performs radiative-transfer calculations to produce models of planetary spectra, and by using a statistical module compares models with observations.