FastChem 4: New chemical elements and improved convergence behaviour
Abstract
Chemical equilibrium calculations are a key ingredient for modelling and interpreting spectroscopic observations of (exo)planets, brown dwarfs, cool stars, and protoplanetary disks. As these applications increasingly probe non-solar elemental abundances and previously underrepresented elements, equilibrium chemistry solvers must be both numerically robust and capable of handling complex chemical systems. Here we present FastChem 4, a major update to the open-source FastChem equilibrium chemistry code. We extend the gas-phase solver with a multidimensional Newton-method that mitigates the slow convergence previously encountered for strongly non-solar elemental abundances. We further reformulate the gas-phase equations in logarithmic element densities, removing the dependence on quad-precision arithmetic and allowing FastChem to be applied at low temperatures on any platform supporting double precision. The condensate solver is upgraded with adaptive Levenberg-Marquardt regularisation, a perturbed-Hessian fallback, and a combined gas-condensate Newton solver. These changes lead to a strong increase in computational performance and stability. The thermochemical data is expanded using thermochemical data from the NIST-JANAF tables and the Barin compilation, and now comprises 800 gas-phase molecules and ions and 511 condensates spanning 44 elements. We apply the updated code to a wide pressure-temperature grid for both solar and carbon-rich (C/O = 2) elemental compositions. The resulting grids reproduce the classical solar-composition condensation sequence and reveal the marked shifts that occur under carbon-rich conditions. We also find that silicon monoxide is stable as a condensate over a limited pressure-temperature range, consistent with recent JWST observations of brown dwarfs. FastChem 4 is released under the GPLv3 licence, together with a pre-compiled Python package.
Related
- HD 152384: A 5-megayear-old debris disk system showing a gaseous ring of refractory elements at the dust sublimation radius
- Reconstructing the aqueous environment of the Ryugu parent body through the speciation of multiple elements
- Simulated Effects of Radioactive Elements on Magnetic Evolution and Atmospheric Mass Loss of Earth-like Planets
- Mg II Signatures of Frequency Dependent Wave Propagation in Magnetic Elements
- The Peculiar Astrometric Behaviour and Radio Structure of the Quasar 1524-136 Revealed with Very Long Baseline Interferometry
BibTeX
@misc{kitzmann2026fastchem,
title = {{FastChem 4: New chemical elements and improved convergence behaviour}},
author = {Daniel Kitzmann and Joachim W. Stock and A. Beate C. Patzer},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2605.18264},
eprint = {2605.18264},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2605.18264},
}
RIS
TY - UNPB
TI - FastChem 4: New chemical elements and improved convergence behaviour
AU - Daniel Kitzmann
AU - Joachim W. Stock
AU - A. Beate C. Patzer
PY - 2026
DA - 2026/11/05
PB - arXiv
DO - 10.48550/arxiv.2605.18264
UR - https://arxiv.org/abs/2605.18264
AB - Chemical equilibrium calculations are a key ingredient for modelling and interpreting spectroscopic observations of (exo)planets, brown dwarfs, cool stars, and protoplanetary disks. As these applications increasingly probe non-solar elemental abundances and previously underrepresented elements, equilibrium chemistry solvers must be both numerically robust and capable of handling complex chemical systems. Here we present FastChem 4, a major update to the open-source FastChem equilibrium chemistry code. We extend the gas-phase solver with a multidimensional Newton-method that mitigates the slow convergence previously encountered for strongly non-solar elemental abundances. We further reformulate the gas-phase equations in logarithmic element densities, removing the dependence on quad-precision arithmetic and allowing FastChem to be applied at low temperatures on any platform supporting double precision. The condensate solver is upgraded with adaptive Levenberg-Marquardt regularisation, a perturbed-Hessian fallback, and a combined gas-condensate Newton solver. These changes lead to a strong increase in computational performance and stability. The thermochemical data is expanded using thermochemical data from the NIST-JANAF tables and the Barin compilation, and now comprises 800 gas-phase molecules and ions and 511 condensates spanning 44 elements. We apply the updated code to a wide pressure-temperature grid for both solar and carbon-rich (C/O = 2) elemental compositions. The resulting grids reproduce the classical solar-composition condensation sequence and reveal the marked shifts that occur under carbon-rich conditions. We also find that silicon monoxide is stable as a condensate over a limited pressure-temperature range, consistent with recent JWST observations of brown dwarfs. FastChem 4 is released under the GPLv3 licence, together with a pre-compiled Python package.
ER -