arxiv: 2203.09491 · v1 · submitted 2022-03-17 · ⚛️ physics.chem-ph

Recognition: no theorem link

Accurate core-excited states via inclusion of core triple excitations in similarity-transformed EOM theory

Megan Simons , Devin A. Matthews

Authors on Pith 1 claimed

Megan Simons ORCID verified

Pith reviewed 2026-05-14 22:07 UTC · model grok-4.3

classification ⚛️ physics.chem-ph

keywords core excitationsK-edge spectroscopyequation-of-motion coupled clustersimilarity-transformed EOMcore-valence separationtriple excitationsorbital relaxation

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The pith

Including triples only in the core ionization step of STEOM yields K-edge excitation energies nearly as accurate as full triples methods.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that orbital relaxation around a core hole dominates errors in linear-response calculations of core excitations. By adding triple excitations solely during the core ionization stage of similarity-transformed EOM-CC, the resulting CVS-STEOMEE-CCSD+cT method recovers most of the accuracy of full CVS-EOM-CCSDT at far lower cost. On K-edge energies for small molecules, both absolute positions and relative spacings improve markedly over standard CVS-EOMEE-CCSD and unmodified STEOM, reaching quality comparable to transition-potential CC and perturbative-triples EOM variants.

Core claim

CVS-STEOMEE-CCSD+cT, which augments CVS-STEOMEE-CCSD by correlating triple excitations only inside the core-ionization step, produces K-edge core-excitation energies whose absolute and shifted errors relative to full CVS-EOM-CCSDT are substantially smaller than those of CVS-EOMEE-CCSD or plain CVS-STEOMEE-CCSD and approach the accuracy of TP-CC and CVS-EOMEE-CCSD*.

What carries the argument

Core-valence-separated similarity-transformed EOM-CC in which triples enter exclusively the core-ionization energy calculation, thereby capturing the dominant orbital relaxation effect.

If this is right

Absolute K-edge peak positions become reliable enough for direct assignment without large empirical shifts.
Relative spacings within a single edge improve enough to resolve close-lying core states in larger molecules.
Computational cost remains closer to CCSD than to full CCSDT while retaining most of the accuracy gain from triples.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

The same selective-triples strategy may transfer to other core spectroscopies such as X-ray absorption or photoemission.
If the core-only triples approximation remains accurate for open-shell or transition-metal systems, routine calculations on larger complexes become feasible.

Load-bearing premise

The dominant relaxation effect on core excitations is captured by triples in the core-ionization step alone, without requiring triples throughout the valence excitation manifold.

What would settle it

Direct numerical comparison of CVS-STEOMEE-CCSD+cT K-edge energies against full CVS-EOM-CCSDT results for a larger set of molecules would show whether the reported error reduction holds or saturates.

read the original abstract

The phenomenon of orbital relaxation upon excitation of core electrons is a major problem in the linear-response treatment of core-hole spectroscopies. Rather than addressing relaxation through direct dynamical correlation of the excited state via equation-of-motion coupled cluster theory (EOMEE-CC), we extend the alternative similarity-transformed equation-of-motion coupled cluster theory (STEOMEE-CC) by including the core-valence separation (CVS) and correlation of triple excitations only within the calculation of core ionization energies. This new method, CVS-STEOMEE-CCSD+cT, significantly improves on CVS-EOMEE-CCSD and unmodified CVS-STEOMEE-CCSD when compared to full CVS-EOM-CCSDT for K-edge core-excitation energies of a set of small molecules. The improvement in both absolute and relative (shifted) peak positions is nearly as good as for transition-potential coupled cluster (TP-CC), which includes an explicit treatment of orbital relaxation, and CVS-EOMEE-CCSD*, which includes a perturbative treatment of triple excitations.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

Selective triples in the core-ionization block of CVS-STEOM give a practical accuracy boost for K-edge energies, but the abstract alone leaves the size of the gain and the implementation details uncheckable.

read the letter

The main advance here is a targeted truncation: triples are kept only inside the core-ionization step of the similarity-transformed EOM, while the subsequent valence excitation block stays at doubles. That choice is new and directly attacks the orbital-relaxation error that hurts plain CVS-EOM-CCSD on core edges. The abstract reports that the resulting CVS-STEOMEE-CCSD+cT numbers sit close to both full CVS-EOM-CCSDT and to TP-CC on a set of small-molecule K-edges, for both absolute and shifted positions. That is a useful engineering result if it survives scrutiny. The method stays cheaper than full triples everywhere, which matters for molecules beyond a handful of atoms. The limitation is obvious from the supplied text: no tables, no basis-set or geometry details, no raw deviations, and no statement of how the triples are actually contracted or screened. Without those numbers it is impossible to judge whether the reported improvement is uniform or whether it degrades on particular edges or larger systems. The central claim is therefore plausible but still provisional. This is the kind of incremental methods paper that belongs in a specialized journal once the benchmarks are shown in full. A serious referee should see it, mainly to check the implementation choices and to ask for a broader test set. I would bring it to a reading group only if someone already works in core spectroscopy or STEOM variants; otherwise the payoff is too narrow. I would not cite it in my own work until the numbers are public and reproducible.

Referee Report

1 major / 0 minor

Summary. The manuscript proposes CVS-STEOMEE-CCSD+cT, which augments similarity-transformed EOM-CC with the core-valence separation and restricts triple excitations to the core-ionization step only. It claims that this yields K-edge core-excitation energies for small molecules that are substantially closer to full CVS-EOM-CCSDT benchmarks than those obtained from CVS-EOMEE-CCSD or unmodified CVS-STEOMEE-CCSD, with accuracy approaching that of transition-potential CC and CVS-EOMEE-CCSD*.

Significance. If the numerical improvements hold, the selective inclusion of core triples offers an economical route to orbital-relaxation effects in core spectroscopies, enabling calculations on systems where full CCSDT remains prohibitive while retaining the formal advantages of the STEOM framework.

major comments (1)

[Abstract] Abstract: the central claim that CVS-STEOMEE-CCSD+cT 'significantly improves' on CVS-EOMEE-CCSD and unmodified CVS-STEOMEE-CCSD relative to CVS-EOM-CCSDT is stated without numerical values, mean absolute errors, standard deviations, basis-set details, geometries, or the molecular test set, rendering the improvement impossible to assess or reproduce from the supplied text.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the report. We address the single major comment below and will revise the manuscript accordingly.

read point-by-point responses

Referee: [Abstract] Abstract: the central claim that CVS-STEOMEE-CCSD+cT 'significantly improves' on CVS-EOMEE-CCSD and unmodified CVS-STEOMEE-CCSD relative to CVS-EOM-CCSDT is stated without numerical values, mean absolute errors, standard deviations, basis-set details, geometries, or the molecular test set, rendering the improvement impossible to assess or reproduce from the supplied text.

Authors: We agree that the abstract would be strengthened by the inclusion of quantitative error metrics. In the revised version we will add the mean absolute errors (with respect to CVS-EOM-CCSDT) for CVS-EOMEE-CCSD, CVS-STEOMEE-CCSD, and CVS-STEOMEE-CCSD+cT, together with a concise statement of the molecular test set and basis sets employed. All geometries, complete statistics, and raw excitation energies already appear in the main text and SI; the abstract revision will simply make these key results visible at the outset. revision: yes

Circularity Check

0 steps flagged

No significant circularity

full rationale

The method is defined by explicit, a-priori truncation choices (CVS plus triples retained only inside the core-ionization step of STEOM). No parameters are fitted to the reported K-edge energies, no self-citation chain is invoked to justify the truncation, and the numerical improvements are presented as direct comparisons against the independent benchmark CVS-EOM-CCSDT. The derivation is therefore self-contained and externally falsifiable.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review; no explicit free parameters, axioms or invented entities are stated. The central approximation (triples only in core ionization) is a domain-specific truncation choice.

pith-pipeline@v0.9.0 · 5456 in / 978 out tokens · 13005 ms · 2026-05-14T22:07:58.491885+00:00 · methodology