{
  "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.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2605.18264",
  "author_total": 3,
  "authors": [
    {
      "name": "Daniel Kitzmann"
    },
    {
      "name": "Joachim W. Stock"
    },
    {
      "name": "A. Beate C. Patzer"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/ayfdmd315wgyp.bib",
  "cited_by": 1,
  "doi": "10.48550/arxiv.2605.18264",
  "fulltext_url": null,
  "id": "ayfdmd315wgyp",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2605.18264",
  "language": null,
  "licence": "LicenseRef-arXiv-nonexclusive-1.0",
  "licence_label": "arXiv licence",
  "licence_source": "article",
  "licence_url": "http://arxiv.org/licenses/nonexclusive-distrib/1.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.EP"
    ],
    [
      "arxiv",
      "astro-ph.IM"
    ],
    [
      "arxiv",
      "astro-ph.SR"
    ],
    [
      "openalex",
      "3103|Stellar, planetary, and galactic studies"
    ]
  ],
  "openalex": "W7161701430",
  "pdf_url": "https://arxiv.org/pdf/2605.18264",
  "pmcid": null,
  "pmid": null,
  "published": "2026-11-05",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/ayfdmd315wgyp.ris",
  "sources": [
    {
      "first_seen": "2026-11-06T18:21:30Z",
      "native_id": "2605.18264",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2605.18264"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.IM",
      "subfield": "instrumentation-methods"
    },
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.EP",
      "subfield": "planetary"
    },
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.SR",
      "subfield": "stellar"
    }
  ],
  "title": "FastChem 4: New chemical elements and improved convergence behaviour",
  "title_text": "FastChem 4: New chemical elements and improved convergence behaviour",
  "type": "preprint",
  "updated_at": "2026-11-09T00:20:33Z",
  "url": "https://research.draconitic.com/paper/ayfdmd315wgyp",
  "venue": "arXiv",
  "versions": []
}