{
  "abstract": "We present a state-specific second-order perturbation theory based on a configuration interaction singles (CIS) reference and its generalized Fock operator. Starting from a fully internally contracted construction, we retain the complete doubles space and internally contracted triples in the first-order wave function. We consider two partitions, Canonical and Block-diagonal, with reference excitation energies given by the Fock excitation gap $ω^{(0)}$ and the CIS excitation energy $ω_{\\rm CIS}$, respectively. Our analysis establishes that strict size-intensivity is preserved when the contracted-triples denominators use bare Fock gaps and the occupied--virtual Fock block is projected to remove residual spectator coupling. Based on this observation, combining Canonical triples with Block-diagonal doubles yields a size-intensive Hybrid partition. These properties are verified for water with non-interacting helium atoms. Our formulation enables $O(o^3v^2)$ cost per matrix--vector product in a semicanonical basis. On the QUEST\\#1 benchmark, Canonical and Block-diagonal show opposite systematic biases, whereas Hybrid gives the best overall accuracy among the three partitions. Its mean absolute errors for singlets and triplets are 0.24 and 0.15~eV, compared with 0.28 and 0.22~eV for CIS(D). The improvement is largest for Rydberg excitations, although CIS(D) remains more accurate for valence singlets.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2610.03583",
  "author_total": 1,
  "authors": [
    {
      "name": "Takashi Tsuchimochi"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/1cxnkjnztbbke.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2610.03583",
  "fulltext_url": null,
  "id": "1cxnkjnztbbke",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2610.03583",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "physics.chem-ph"
    ],
    [
      "openalex",
      "3107|Advanced Chemical Physics Studies"
    ]
  ],
  "openalex": "W7220473266",
  "pdf_url": "https://arxiv.org/pdf/2610.03583",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-02",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/1cxnkjnztbbke.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:26:45Z",
      "native_id": "2610.03583",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2610.03583"
    }
  ],
  "subjects": [
    {
      "field": "chemistry",
      "origin": "arxiv:physics.chem-ph",
      "subfield": "physical"
    }
  ],
  "title": "CIS(2): a state-specific, size-intensive perturbative correction to configuration interaction singles",
  "title_text": "CIS(2): a state-specific, size-intensive perturbative correction to configuration interaction singles",
  "type": "preprint",
  "updated_at": "2026-10-30T12:26:45Z",
  "url": "https://research.draconitic.com/paper/1cxnkjnztbbke",
  "venue": "arXiv",
  "versions": []
}