FastChem: A computer program for efficient complex chemical equilibrium calculations in the neutral/ionized gas phase with applications to stellar and planetary atmospheres

Abstract

For the calculation of complex neutral/ionized gas phase chemical equilibria,\nwe present a semi-analytical versatile and efficient computer program, called\nFastChem. The applied method is based on the solution of a system of coupled\nnonlinear (and linear) algebraic equations, namely the law of mass action and\nthe element conservation equations including charge balance, in many variables.\nSpecifically, the system of equations is decomposed into a set of coupled\nnonlinear equations in one variable each, which are solved analytically\nwhenever feasible to reduce computation time. Notably, the electron density is\ndetermined by using the method of Nelder and Mead at low temperatures. The\nprogram is written in object-oriented C++ which makes it easy to couple the\ncode with other programs, although a stand-alone version is provided. FastChem\ncan be used in parallel or sequentially and is available under the GNU General\nPublic License version 3 at https://github.com/exoclime/FastChem together with\nseveral sample applications. The code has been successfully validated against\nprevious studies and its convergence behavior has been tested even for extreme\nphysical parameter ranges down to 100 K and up to 1000 bar. FastChem converges\nstable and robust in even most demanding chemical situations, which posed\nsometimes extreme challenges for previous algorithms.\n