{
  "abstract": "Rigid-body flow matching reduces a molecular crystal to a lattice, fractional cen-troids and per-molecule orientations on SO(3), and is an attractive route to structure determination from powder X-ray diffraction (PXRD). We report a negative result and its root cause. Our PXRD-conditioned rigid-body flow recovered 0 of 160 held-out structures, and the failure was neither the lattice nor the sampler: a frame-preserving oracle substitution shows that installing the ground-truth unit cell into the generated packing yields 0/10 matches, while a component decomposition on held-out data puts the lattice head at 91.6% of variance explained and both packing heads at their no-information floors (centroid −0.1%, orientation 3.7%). The orientation loss sat at ∼4.7 against an analytic predict-nothing floor of π2/3+2 = 5.290, because the training target was the absolute molecular pose, which factorises as",
  "abstract_shown": true,
  "affiliations": [
    "Purdue University West Lafayette"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 1,
  "authors": [
    {
      "affs": [
        0
      ],
      "name": "Frank Cai",
      "orcid": "0009-0003-0041-1459"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/5efg5c0gkykev.bib",
  "cited_by": 0,
  "doi": "10.26434/chemrxiv.15008251/v1",
  "fulltext_url": null,
  "id": "5efg5c0gkykev",
  "issns": [],
  "landing_url": "https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008251/v1",
  "language": null,
  "licence": "CC-BY",
  "licence_label": "CC BY",
  "licence_source": "journal",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [
    [
      "openalex",
      "2505|X-ray Diffraction in Crystallography"
    ]
  ],
  "openalex": "W7207678806",
  "pdf_url": "https://chemrxiv.org/doi/pdf/10.26434/chemrxiv.15008251/v1",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-03",
  "publisher": "American Chemical Society (ACS)",
  "ris": "https://research.draconitic.com/paper/5efg5c0gkykev.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:17:17Z",
      "native_id": "10.26434/chemrxiv.15008251",
      "source": "chemrxiv",
      "url": "https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008251/v1"
    }
  ],
  "subjects": [
    {
      "field": "chemistry",
      "origin": "openalex:2505|X-ray Diffraction in Crystallography",
      "subfield": "materials-polymers"
    }
  ],
  "title": "An unlearnable target: diagnosing the orientation gauge in PXRD-conditioned molecular-crystal flow matching",
  "title_text": "An unlearnable target: diagnosing the orientation gauge in PXRD-conditioned molecular-crystal flow matching",
  "type": "preprint",
  "updated_at": "2026-10-30T12:17:17Z",
  "url": "https://research.draconitic.com/paper/5efg5c0gkykev",
  "venue": "ChemRxiv",
  "versions": []
}