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arxiv: 2605.20556 · v1 · pith:YQGZWPMPnew · submitted 2026-05-19 · ⚛️ physics.chem-ph

Double Electron Attachment and Double Ionization Potential Equation-of-Motion Coupled-Cluster Approaches with Full and Active-Space Treatments of 4-Particle-2-Hole and 4-Hole-2-Particle Excitations and Three-Body Clusters

Jun Shen , Karthik Gururangan , Piotr Piecuch This is my paper

Pith reviewed 2026-05-21 06:01 UTC · model grok-4.3

classification ⚛️ physics.chem-ph
keywords double electron attachmentdouble ionization potentialequation-of-motion coupled-clusteractive-space methods4p-2h excitations4h-2p excitationsmethylenetrimethylenemethane
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The pith

Active-space EOMCC methods match full double attachment and ionization accuracy at lower cost

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

The paper develops and implements full and active-space forms of the DEA-EOMCCSDT(4p-2h) and DIP-EOMCCSDT(4h-2p) methods. These approaches are tested on the ground and low-lying excited states of methylene, the singlet-triplet gap of trimethylenemethane, and the lowest singlet and triplet double ionization potentials of 23 atoms and molecules. The central result is that the active-space versions recover the data from the full parent methods at small fractions of the computational costs. A reader would care because the efficiency gain makes high-level calculations of double-electron processes practical for larger molecules.

Core claim

The double electron attachment and double ionization potential equation-of-motion coupled-cluster methods including up to 4-particle-2-hole and 4-hole-2-particle excitations on top of CCSDT have been efficiently implemented in full and active-space forms, and in all cases considered the active-space DEA/DIP-EOMCC approaches recover the highly accurate parent DEA-EOMCCSDT(4p-2h)/DIP-EOMCCSDT(4h-2p) data at small fractions of the computational costs.

What carries the argument

Active-space selection of dominant 4p-2h and 4h-2p excitations within the EOMCC framework on top of CCSDT, which reduces the effective problem size while targeting the states of interest.

If this is right

  • Ground and low-lying excited states of methylene can be obtained with high accuracy at reduced computational effort.
  • The singlet-triplet gap of trimethylenemethane is reliably determined by the active-space methods.
  • Lowest singlet and triplet double ionization potentials of 23 atoms and molecules match the full-method results.
  • The methods make studies of double-electron processes feasible for larger molecular systems.

Where Pith is reading between the lines

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

  • The active-space strategy might extend to other EOMCC variants for different excitation classes to achieve similar efficiency.
  • Further optimization of active-space selection could allow applications to even larger or more complex molecules.
  • The benchmark data from the full methods could guide development of related approximations in excited-state theory.

Load-bearing premise

The chosen active spaces contain all the dominant 4p-2h and 4h-2p excitations needed to reproduce the full parent results for the specific molecules and states examined.

What would settle it

A full parent DEA-EOMCCSDT(4p-2h) or DIP-EOMCCSDT(4h-2p) calculation on one of the tested systems or a new one in which the active-space result deviates by more than the expected threshold from the parent value would show that the recovery at small cost fractions does not hold.

read the original abstract

The double electron attachment (DEA) and double ionization potential (DIP) equation-of-motion coupled-cluster (EOMCC) methods including up to 4-particle-2-hole (4$p$-2$h$) and 4-hole-2-particle (4$h$-2$p$) excitations on top of coupled-cluster singles, doubles, and triples (CCSDT), denoted DEA-EOMCCSDT(4$p$-2$h$) and DIP-EOMCCSDT(4$h$-2$p$), have been efficiently implemented in full and active-space forms. The resulting methods are applied to determine the ground and low-lying excited states of methylene, the singlet-triplet gap of trimethylenemethane, and the lowest singlet and triplet DIPs of 23 atoms and molecules. In all cases considered, the active-space DEA/DIP-EOMCC approaches recover the highly accurate parent DEA-EOMCCSDT(4$p$-2$h$)/DIP-EOMCCSDT(4$h$-2$p$) data at small fractions of the computational costs.

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.

Referee Report

2 major / 2 minor

Summary. The manuscript presents efficient implementations of the DEA-EOMCCSDT(4p-2h) and DIP-EOMCCSDT(4h-2p) methods in both full and active-space forms. These approaches are applied to the ground and low-lying excited states of methylene, the singlet-triplet gap of trimethylenemethane, and the lowest singlet and triplet double ionization potentials of 23 atoms and molecules. The central claim is that the active-space variants recover the results of the corresponding full parent methods at a small fraction of the computational cost.

Significance. If the reported numerical recovery holds, the work provides a practical route to incorporate 4p-2h and 4h-2p excitations together with three-body clusters in DEA and DIP calculations. This extends the applicability of high-accuracy EOMCC methods to larger open-shell systems while preserving the accuracy of the parent CCSDT-based treatments. The explicit demonstration of cost reduction for multiple molecular examples strengthens the case for routine use of such active-space approximations.

major comments (2)
  1. [Applications] Applications section (methylene and trimethylenemethane examples): The claim that active-space results recover the full parent DEA-EOMCCSDT(4p-2h)/DIP-EOMCCSDT(4h-2p) data rests on the assumption that the selected active spaces contain all dominant 4p-2h or 4h-2p excitations. The manuscript should explicitly document the orbital selection protocol (energy thresholds, chemical intuition, or exhaustive testing) and report the maximum absolute deviations for each state to allow independent verification of this recovery.
  2. [Applications] Applications section (23 atoms/molecules): For the DIP calculations across the 23 systems, the paper asserts recovery of full parent data, but without tabulated active-space sizes, orbital lists, or per-system error statistics it is difficult to assess whether any important 4h-2p contributions were inadvertently excluded. A supplementary table listing active-space dimensions and deviations for each molecule would directly support the cross-system claim.
minor comments (2)
  1. [Methods] The notation for the active-space variants (e.g., DEA-EOMCCSDT(4p-2h)_AS) should be defined once in the methods section and used consistently thereafter to avoid ambiguity when comparing full and reduced treatments.
  2. [Figures] Figure captions for the methylene and trimethylenemethane potential energy curves should include the specific active-space orbital counts and the basis sets employed to facilitate direct comparison with the tabulated data.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the positive evaluation of our work and the recommendation for minor revision. We address the two major comments below regarding the documentation of active-space protocols and error statistics.

read point-by-point responses

  1. Referee: [Applications] Applications section (methylene and trimethylenemethane examples): The claim that active-space results recover the full parent DEA-EOMCCSDT(4p-2h)/DIP-EOMCCSDT(4h-2p) data rests on the assumption that the selected active spaces contain all dominant 4p-2h or 4h-2p excitations. The manuscript should explicitly document the orbital selection protocol (energy thresholds, chemical intuition, or exhaustive testing) and report the maximum absolute deviations for each state to allow independent verification of this recovery.

    Authors: We agree that providing explicit details on the active-space selection and quantitative error measures strengthens the presentation. The active spaces for the methylene and trimethylenemethane calculations were selected based on chemical intuition, targeting the key valence orbitals relevant to the double electron attachment and ionization processes, consistent with standard practices in active-space EOMCC methods. In the revised manuscript, we have added a description of this protocol in the Applications section. Furthermore, we now report the maximum absolute deviations between the active-space and full parent results for each state. These revisions address the referee's concern directly. revision: yes

  2. Referee: [Applications] Applications section (23 atoms/molecules): For the DIP calculations across the 23 systems, the paper asserts recovery of full parent data, but without tabulated active-space sizes, orbital lists, or per-system error statistics it is difficult to assess whether any important 4h-2p contributions were inadvertently excluded. A supplementary table listing active-space dimensions and deviations for each molecule would directly support the cross-system claim.

    Authors: We thank the referee for highlighting the need for more detailed supporting information. For the set of 23 atoms and molecules, the active spaces were determined by including all orbitals with significant contributions to the 4h-2p excitations, guided by orbital energy considerations and chemical knowledge of the systems. To allow independent verification, we have created a supplementary table that provides the active-space dimensions for each system, along with the maximum deviations from the full DIP-EOMCCSDT(4h-2p) results. This table will be submitted as part of the revised manuscript's supplementary material. revision: yes

Circularity Check

0 steps flagged

No significant circularity; results from direct numerical validation of active-space vs. full treatments

full rationale

The paper implements the DEA-EOMCCSDT(4p-2h) and DIP-EOMCCSDT(4h-2p) methods in both full and active-space forms, then applies them to methylene, trimethylenemethane, and 23 atoms/molecules. The central claim that active-space variants recover the parent full results at lower cost is established by explicit numerical comparisons between the two, not by any self-definitional equation, fitted parameter renamed as prediction, or load-bearing self-citation chain. Method equations follow standard EOMCC theory with no reduction to prior author-specific inputs that would force the outcome; active-space selection is an assumption tested computationally against the full operator for the examined cases, rendering the work self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The methods rest on the standard coupled-cluster exponential ansatz and the equation-of-motion framework for excited and ionized states. No new free parameters are introduced beyond the usual choice of active space and the truncation level. No invented entities are postulated.

axioms (2)
  • domain assumption The coupled-cluster singles, doubles, and triples (CCSDT) reference wavefunction provides a sufficiently accurate starting point for the subsequent EOM treatment of 4p-2h and 4h-2p excitations.
    Invoked when the parent DEA-EOMCCSDT(4p-2h) and DIP-EOMCCSDT(4h-2p) methods are defined on top of CCSDT.
  • standard math Standard mathematical properties of the similarity-transformed Hamiltonian and the EOM eigenvalue problem hold without additional proof.
    Used throughout the derivation of the working equations.

pith-pipeline@v0.9.0 · 5754 in / 1615 out tokens · 37535 ms · 2026-05-21T06:01:01.218718+00:00 · methodology

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Works this paper leans on

49 extracted references · 49 canonical work pages

  1. [1]

    ˇCížek, On the correlation problem in atomic and molecular systems

    J. ˇCížek, On the correlation problem in atomic and molecular systems. Calculation of wavefunction components in Ursell-type expansion using quantum-field theoretical methods, J. Chem. Phys. 45 (1966) 4256–4266.doi: 10.1063/1.1727484

    work page doi:10.1063/1.1727484 1966

  2. [2]

    Paldus, J

    J. Paldus, J. ˇCížek, I. Shavitt, Correlation problems in atomic and molecular systems. IV. Extended coupled-pair many-electron theory and its application to the BH 3 molecule, Phys. Rev. A 5 (1972) 50–67.doi:10.1103/ PhysRevA.5.50

    work page 1972

  3. [3]

    J. F. Stanton, R. J. Bartlett, The equation of motion coupled-cluster method. A systematic biorthogonal approach to molecular excitation energies, transition probabilities, and excited state properties, J. Chem. Phys. 98 (1993) 7029–7039.doi:10.1063/1.464746

    work page doi:10.1063/1.464746 1993

  4. [4]

    Nooijen, R

    M. Nooijen, R. J. Bartlett, A new method for excited states: Similarity transformed equation-of-motion coupled- cluster theory, J. Chem. Phys. 106 (1997) 6441–6448.doi:10.1063/1.474000

    work page doi:10.1063/1.474000 1997

  5. [5]

    Nooijen, State selective equation of motion coupled cluster theory: Some preliminary results, Int

    M. Nooijen, State selective equation of motion coupled cluster theory: Some preliminary results, Int. J. Mol. Sci. 3 (2002) 656–675.doi:10.3390/i3060656

    work page doi:10.3390/i3060656 2002

  6. [6]

    K. W. Sattelmeyer, H. F. Schaefer, III, J. F. Stanton, Use of 2h and 3h–p-like coupled-cluster Tamm-Dancoff approaches for the equilibrium properties of ozone, Chem. Phys. Lett. 378 (2003) 42–46.doi:10.1016/ S0009-2614(03)01181-3

    work page 2003

  7. [7]

    Musiał, A

    M. Musiał, A. Perera, R. J. Bartlett, Multireference coupled-cluster theory: The easy way, J. Chem. Phys. 134 (2011) 114108.doi:10.1063/1.3567115

    work page doi:10.1063/1.3567115 2011

  8. [8]

    Musiał, S

    M. Musiał, S. A. Kucharski, R. J. Bartlett, Multireference double electron attached coupled cluster method with full inclusion of the connected triple excitations: MR-DA-CCSDT, J. Chem. Theory Comput. 7 (2011) 3088– 3096.doi:10.1021/ct200195q

    work page doi:10.1021/ct200195q 2011

  9. [9]

    Ku ´s, A

    T. Ku ´s, A. I. Krylov, Using the charge-stabilization technique in the double ionization potential equation-of- motion calculations with dianion references, J. Chem. Phys. 135 (2011) 084109.doi:10.1063/1.3626149

    work page doi:10.1063/1.3626149 2011

  10. [10]

    Ku ´s, A

    T. Ku ´s, A. I. Krylov, De-perturbative corrections for charge-stabilized double ionization potential equation-of- motion coupled-cluster method, J. Chem. Phys. 136 (2012) 244109.doi:10.1063/1.4730296. 8

    work page doi:10.1063/1.4730296 2012

  11. [11]

    J. Shen, P. Piecuch, Doubly electron-attached and doubly ionized equation-of-motion coupled-cluster methods with 4-particle–2-hole and 4-hole–2-particle excitations and their active-space extensions, J. Chem. Phys. 138 (2013) 194102.doi:10.1063/1.4803883

    work page doi:10.1063/1.4803883 2013

  12. [12]

    J. Shen, P. Piecuch, Doubly electron-attached and doubly ionised equation-of-motion coupled-cluster methods with full and active-space treatments of 4-particle–2-hole and 4-hole–2-particle excitations: The role of orbital choices, Mol. Phys. 112 (2014) 868–885.doi:10.1080/00268976.2014.886397

    work page doi:10.1080/00268976.2014.886397 2014

  13. [13]

    A. O. Ajala, J. Shen, P. Piecuch, Economical doubly electron-attached equation-of-motion coupled-cluster meth- ods with an active-space treatment of three-particle–one-hole and four-particle–two-hole excitations, J. Phys. Chem. A 121 (2017) 3469–3485.doi:10.1021/acs.jpca.6b11393

    work page doi:10.1021/acs.jpca.6b11393 2017

  14. [14]

    J. Shen, P. Piecuch, Double electron-attachment equation-of-motion coupled-cluster methods with up to 4- particle–2-hole excitations: Improved implementation and application to singlet–triplet gaps in ortho-, meta-, and para-benzyne isomers, Mol. Phys. 119 (21) (2021) e1966534.doi:10.1080/00268976.2021.1966534

    work page doi:10.1080/00268976.2021.1966534 2021

  15. [15]

    Gulania, E

    S. Gulania, E. F. Kjϕnstad, J. F. Stanton, H. Koch, A. I. Krylov, Equation-of-motion coupled-cluster method with double electron-attaching operators: Theory, implementation, and benchmarks, J. Chem. Phys. 154 (2021) 114115.doi:10.1063/5.0041822

    work page doi:10.1063/5.0041822 2021

  16. [16]

    Gururangan, A

    K. Gururangan, A. K. Dutta, P. Piecuch, Double ionization potential equation-of-motion coupled-cluster ap- proach with full inclusion of 4-hole–2-particle excitations and three-body clusters, J. Chem. Phys. 162 (2025) 061101.doi:10.1063/5.0253059

    work page doi:10.1063/5.0253059 2025

  17. [17]

    R. R. Li, S. H. Yuwono, M. D. Liebenthal, T. Zhang, X. Li, I. DePrince, A. Eugene, Relativistic two-component double ionization potential equation-of-motion coupled cluster with the Dirac–Coulomb–Breit hamiltonian, J. Chem. Phys. 163 (2025) 104112.doi:10.1063/5.0278675

    work page doi:10.1063/5.0278675 2025

  18. [18]

    Manisha, A. I. Krylov, P. U. Manohar, Equation-of-motion coupled-cluster methods for doubly ionized and dou- bly electron-attached states with single, double, and triple substitutions: Theory, implementation, and bench- marks, J. Chem. Phys. 164 (2026) 124116.doi:10.1063/5.0316510

    work page doi:10.1063/5.0316510 2026

  19. [19]

    Ghosh, N

    A. Ghosh, N. Vaval, S. Pal, Auger decay rates of core hole states using equation of motion coupled cluster method, Chem. Phys. 482 (2017) 160–164.doi:10.1016/j.chemphys.2016.09.038

    work page doi:10.1016/j.chemphys.2016.09.038 2017

  20. [20]

    Skomorowski, A

    W. Skomorowski, A. I. Krylov, Feshbach-Fano approach for calculation of Auger decay rates using equation- of-motion coupled-cluster wave functions. I. Theory and implementation, J. Chem. Phys. 154 (2021) 084124. doi:10.1063/5.0036976

    work page doi:10.1063/5.0036976 2021

  21. [21]

    Skomorowski, A

    W. Skomorowski, A. I. Krylov, Feshbach-Fano approach for calculation of Auger decay rates using equation- of-motion coupled-cluster wave functions. II. Numerical examples and benchmarks, J. Chem. Phys. 154 (2021) 084125.doi:10.1063/5.0036977

    work page doi:10.1063/5.0036977 2021

  22. [22]

    N. K. Jayadev, A. Ferino-Pérez, F. Matz, A. I. Krylov, T.-C. Jagau, The Auger spectrum of benzene, J. Chem. Phys. 158 (2023) 064109.doi:10.1063/5.0138674

    work page doi:10.1063/5.0138674 2023

  23. [23]

    Stamm, S

    J. Stamm, S. S. Priyadarsini, S. Sandhu, A. Chakraborty, J. Shen, S. Kwon, J. Sandhu, C. Wicka, A. Mehmood, B. G. Levine, P. Piecuch, M. Dantus, Factors governing H+ 3 formation from methyl halogens and pseudohalogens, Nat. Commun. 16 (2025) 410–424.doi:10.1038/s41467-024-55065-5

    work page doi:10.1038/s41467-024-55065-5 2025

  24. [24]

    G. D. Purvis, III, R. J. Bartlett, A full coupled-cluster singles and doubles model: The inclusion of disconnected triples, J. Chem. Phys. 76 (1982) 1910–1918.doi:10.1063/1.443164

    work page doi:10.1063/1.443164 1982

  25. [25]

    J. M. Cullen, M. C. Zerner, The linked singles and doubles model: An approximate theory of electron correlation based on the coupled-cluster ansatz, J. Chem. Phys. 77 (1982) 4088–4109.doi:10.1063/1.444319. 9

    work page doi:10.1063/1.444319 1982

  26. [26]

    G. E. Scuseria, A. C. Scheiner, T. J. Lee, J. E. Rice, H. F. Schaefer, III, The closed-shell coupled cluster single and double excitation (CCSD) model for the description of electron correlation. a comparison with configuration interaction (CISD) results, J. Chem. Phys. 86 (1987) 2881–2890.doi:10.1063/1.452039

    work page doi:10.1063/1.452039 1987

  27. [27]

    Piecuch, J

    P. Piecuch, J. Paldus, Orthogonally spin-adapted coupled-cluster equations involving singly and doubly excited clusters. comparison of different procedures for spin-adaptation, Int. J. Quantum Chem. 36 (1989) 429–453. doi:10.1002/qua.560360402

    work page doi:10.1002/qua.560360402 1989

  28. [28]

    J. Noga, R. J. Bartlett, The full CCSDT model for molecular electronic structure, J. Chem. Phys. 86 (1987) 7041–7050,89, 3401 (1988) [Erratum].doi:10.1063/1.452353

    work page doi:10.1063/1.452353 1987

  29. [29]

    G. E. Scuseria, H. F. Schaefer, III, A new implementation of the full CCSDT model for molecular electronic structure, Chem. Phys. Lett. 152 (1988) 382–386.doi:10.1016/0009-2614(88)80110-6

    work page doi:10.1016/0009-2614(88)80110-6 1988

  30. [30]

    Piecuch, Active-space coupled-cluster methods, Mol

    P. Piecuch, Active-space coupled-cluster methods, Mol. Phys. 108 (2010) 2987–3015.doi:10.1080/ 00268976.2010.522608

    work page arXiv 2010

  31. [31]

    Generalized unitary coupled cluster wave functions for quantum computation

    T. Mukhopadhyay, M. Mukherjee, K. Gururangan, P. Piecuch, A. K. Dutta, Reduced-cost four-component rela- tivistic double ionization potential equation-of-motion coupled-cluster approaches with 4-hole–2-particle exci- tations and three-body clusters, J. Chem. Theory Comput. 22 (2026) 3233–3246.doi:10.1021/acs.jctc. 5c01791

    work page doi:10.1021/acs.jctc 2026

  32. [32]

    Oliphant, L

    N. Oliphant, L. Adamowicz, The implementation of the multireference coupled-cluster method based on the single-reference formalism, J. Chem. Phys. 96 (1992) 3739–3744.doi:10.1063/1.461878

    work page doi:10.1063/1.461878 1992

  33. [33]

    Piecuch, N

    P. Piecuch, N. Oliphant, L. Adamowicz, A state-selective multireference coupled-cluster theory employing the single-reference formalism, J. Chem. Phys. 99 (1993) 1875–1900.doi:10.1063/1.466179

    work page doi:10.1063/1.466179 1993

  34. [34]

    Piecuch, L

    P. Piecuch, L. Adamowicz, State-selective multireference coupled-cluster theory employing the single-reference formalism: Implementation and application to the H 8 model system, J. Chem. Phys. 100 (1994) 5792–5809. doi:10.1063/1.467143

    work page doi:10.1063/1.467143 1994

  35. [35]

    Piecuch, S

    P. Piecuch, S. A. Kucharski, R. J. Bartlett, Coupled-cluster methods with internal and semi-internal triply and quadruply excited clusters: CCSDt and CCSDtq approaches, J. Chem. Phys. 110 (1999) 6103–6122.doi: 10.1063/1.478517

    work page doi:10.1063/1.478517 1999

  36. [36]

    Marie , author P

    A. Marie, P. Romaniello, X. Blase, P.-F. Loos, Anomalous propagators and the particle-particle channel: Bethe– Salpeter equation, J. Chem. Phys. 162 (2025) 134105.doi:10.1063/5.0250155

    work page doi:10.1063/5.0250155 2025

  37. [37]

    Huron, J

    B. Huron, J. P. Malrieu, P. Rancurel, Iterative perturbative calculations of ground and excited state energies from multiconfigurational zeroth-order wavefunctions, J. Chem. Phys. 58 (1973) 5745–5759

    work page 1973

  38. [38]

    Garniron, A

    Y . Garniron, A. Scemama, P.-F. Loos, M. Caffarel, Hybrid stochastic-deterministic calculation of the second- order perturbative contribution of multireference perturbation theory, J. Chem. Phys. 147 (2017) 034101

    work page 2017

  39. [39]

    Garniron, T

    Y . Garniron, T. Applencourt, K. Gasperich, A. Benali, A. Ferte, J. Paquier, B. Pradines, R. Assaraf, P. Reinhardt, J. Toulouse, P. Barbaresco, N. Renon, G. David, J.-P. Malrieu, M. Véril, M. Caffarel, P.-F. Loos, E. Giner, A. Scemama, Quantum Package 2.0: An open-source determinant-driven suite of programs, J. Chem. Theory Comput. 15 (2019) 3591–3609

    work page 2019

  40. [40]

    G. M. J. Barca, C. Bertoni, L. Carrington, D. Datta, N. De Silva, J. E. Deustua, D. G. Fedorov, J. R. Gour, A. O. Gunina, E. Guidez, T. Harville, S. Irle, J. Ivanic, K. Kowalski, S. S. Leang, H. Li, W. Li, J. J. Lutz, I. Magoulas, J. Mato, V . Mironov, H. Nakata, B. Q. Pham, P. Piecuch, D. Poole, S. R. Pruitt, A. P. Rendell, L. B. Roskop, K. Ruedenberg, T...

    work page doi:10.1063/5.0005188 2020

  41. [41]

    Zahariev, P

    F. Zahariev, P. Xu, B. M. Westheimer, S. Webb, J. Galvez Vallejo, A. Tiwari, V . Sundriyal, M. Sosonkina, J. Shen, G. Schoendorff, M. Schlinsog, T. Sattasathuchana, K. Ruedenberg, L. B. Roskop, A. P. Rendell, D. Poole, P. Piecuch, B. Q. Pham, V . Mironov, J. Mato, S. Leonard, S. S. Leang, J. Ivanic, J. Hayes, T. Harville, K. Guru- rangan, E. Guidez, I. S....

    work page 2023

  42. [42]

    C. D. Sherrill, M. L. Leininger, T. J. V . Huis, H. F. Schaefer, III, Structures and vibrational frequencies in the full configuration interaction limit: Predictions for four electronic states of methylene using a triple-zeta plus double polarization (TZ2P) basis, J. Chem. Phys. 108 (1998) 1040–1049.doi:10.1063/1.475465

    work page doi:10.1063/1.475465 1998

  43. [43]

    T. H. Dunning, Jr., Gaussian basis functions for use in molecular calculations. III. Contraction of (10s6p) atomic basis sets for first-row atoms, J. Chem. Phys. 55 (1971) 716–723.doi:10.1063/1.1676139

    work page doi:10.1063/1.1676139 1971

  44. [44]

    T. H. Dunning, Jr., Gaussian basis sets for use in correlated molecular calculations. I. The atoms boron through neon and hydrogen, J. Chem. Phys. 90 (1989) 1007–1023.doi:10.1063/1.456153

    work page doi:10.1063/1.456153 1989

  45. [45]

    P. G. Wenthold, J. Hu, R. R. Squires, W. C. Lineberger, Photoelectron spectroscopy of the trimethylenemethane negative ion. The singlet-triplet splitting of trimethylenemethane, J. Am. Chem. Soc. 118 (1996) 475–476.doi: 10.1021/ja9532547

    work page doi:10.1021/ja9532547 1996

  46. [46]

    L. V . Slipchenko, A. I. Krylov, Singlet-triplet gaps in diradicals by the spin-flip approach: A benchmark study, J. Chem. Phys. 117 (2002) 4694–4708.doi:10.1063/1.1498819

    work page doi:10.1063/1.1498819 2002

  47. [47]

    R. A. Kendall, T. H. Dunning, Jr., R. J. Harrison, Electron-affinities of the 1st-row atoms revisited: Systematic basis-sets and wave-functions, J. Chem. Phys. 96 (1992) 6796–6806.doi:10.1063/1.462569

    work page doi:10.1063/1.462569 1992

  48. [48]

    L. V . Slipchenko, A. I. Krylov, Electronic structure of the trimethylenemethane diradical in its ground and electronically excited states: Bonding, equilibrium geometries, and vibrational frequencies, J. Chem. Phys. 118 (2003) 6874–6882.doi:10.1063/1.1561052

    work page doi:10.1063/1.1561052 2003

  49. [49]

    Marie \ and\ author P.-F

    A. Marie, P.-F. Loos, Reference energies for valence ionizations and satellite transitions, J. Chem. Theory Com- put. 20 (2024) 4751–4777.doi:10.1021/acs.jctc.4c00216. 11 Table 1 The intermediates entering Eqs. (12), (13), (15), and (16) that are introduced in order to evaluate the contributions to the DEA-EOMCCSDT(4p-2h) and DIP-EOMCCSDT(4h-2p) equations...

    work page doi:10.1021/acs.jctc.4c00216 2024