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| Main Authors: | , , |
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| Format: | Preprint |
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2026
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| Online Access: | https://arxiv.org/abs/2605.18264 |
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| _version_ | 1866917508000251904 |
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| author | Kitzmann, Daniel Stock, Joachim W. Patzer, A. Beate C. |
| author_facet | Kitzmann, Daniel Stock, Joachim W. Patzer, A. Beate C. |
| contents | 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2605_18264 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | FastChem 4: New chemical elements and improved convergence behaviour Kitzmann, Daniel Stock, Joachim W. Patzer, A. Beate C. Earth and Planetary Astrophysics Instrumentation and Methods for Astrophysics Solar and Stellar Astrophysics 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. |
| title | FastChem 4: New chemical elements and improved convergence behaviour |
| topic | Earth and Planetary Astrophysics Instrumentation and Methods for Astrophysics Solar and Stellar Astrophysics |
| url | https://arxiv.org/abs/2605.18264 |