easyCHEM: A Python package for calculating chemical equilibrium abundances in exoplanet atmospheres

Abstract: For modeling the spectra of exoplanets one must know their atmospheric composition. This is necessary because the abundance of molecules, atoms, ions and condensates is needed to construct the total cross-section for the interaction between electro-magnetic radiation and matter. In addition, when solving for the temperature structure of an atmosphere the so-called adiabatic temperature gradient must be known, which describes the pressure-temperature dependence in convectively unstable regions well. Depending on the planetary properties, the composition and adiabatic gradients may be well described by equilibrium chemistry, which means that chemical reactions occur faster than any other relevant processes in the atmosphere, such as mixing. What is more, the equilibrium assumption often serves as a useful starting point for non-equilibrium calculations. Efficient and easy-to-use codes for determining equilibrium abundances are therefore needed. Here we report on our easyCHEM Python package that calculates atmospheric compositions and adiabatic temperature gradients in chemical equilibrium for any user-specified elemental composition.