{
  "abstract": "Van Aggelen et al. [Phys. Chem. Chem. Phys. 11, 5558 (2009)] showed that variational second-order reduced-density-matrix (2-RDM) calculations with the standard two-index conditions dissociate diatomic molecules into fractionally charged atoms with energies far below the exact limit, a drastic violation of size consistency; the proposed subsystem constraints require auxiliary calculations with the physical fragment Hamiltonians. We show that two long-known ingredients, combined, remove most of this failure without such calculations: the requirement that the number of electrons in a part of the molecule be an integer, imposed as linear conditions on the 2-RDM, and a chemically motivated partition of the molecule into atomic regions bounded at the bond critical point of the Hartree-Fock density. In a minimal basis, compared with full configuration interaction, a single such condition reduces the energy errors of NO$^+$ and CN$^-$ at 6 Å from 108.9 and 80.7 m$E_\\mathrm{h}$ to less than 0.02 m$E_\\mathrm{h}$. In N$_2^+$, whose fragments have the correct mean population but too small a fluctuation, the region and three-index conditions act complementarily, reducing the error at 4 Å from 138.0 to 1.6 m$E_\\mathrm{h}$. At the separated-atom limit, the region cuts make the $P$, $Q$, $G$ energy additive over the fragments to within 0.2 m$E_\\mathrm{h}$ in 7 of 10 systems, 3 already nearly additive without cuts; adding the $T_1$ and $T_2$ conditions also makes CO nearly additive, while a nonadditive error remains for N$_2^+$ and CO$^+$. Effective partitions are those whose condition is nearly saturated by the exact state.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2610.36514",
  "author_total": 1,
  "authors": [
    {
      "name": "Maho Nakata"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/e8d9x01p015w0.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2610.36514",
  "fulltext_url": null,
  "id": "e8d9x01p015w0",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2610.36514",
  "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",
      "physics.chem-ph"
    ],
    [
      "openalex",
      "3107|Advanced Chemical Physics Studies"
    ]
  ],
  "openalex": "W7224127931",
  "pdf_url": "https://arxiv.org/pdf/2610.36514",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-27",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/e8d9x01p015w0.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:35:50Z",
      "native_id": "2610.36514",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2610.36514"
    }
  ],
  "subjects": [
    {
      "field": "chemistry",
      "origin": "arxiv:physics.chem-ph",
      "subfield": "physical"
    }
  ],
  "title": "Integer electron-count cuts in variational second-order reduced-density-matrix theory. I. Molecular dissociation with fixed real-space and orbital partitions",
  "title_text": "Integer electron-count cuts in variational second-order reduced-density-matrix theory. I. Molecular dissociation with fixed real-space and orbital partitions",
  "type": "preprint",
  "updated_at": "2026-10-30T12:35:50Z",
  "url": "https://research.draconitic.com/paper/e8d9x01p015w0",
  "venue": "arXiv",
  "versions": []
}