Valence bond description of antiferromagnetic coupling in transition metal dimers

Noodleman, Louis

doi:10.1063/1.440939

Cited by 2,454 publications

(1,871 citation statements)

References 26 publications

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“…In the later two calculations, we have always checked if a broken-symmetry solution exists, 18,19 to allow the system falling into the open-shell singlet state when it is more stable. It is known that the B3LYP potential energy surface computed using the broken-symmetry approximation mimics that obtained in CASSCF calculations.…”

Section: Methodsmentioning

confidence: 99%

The origin of the two‐electron/four‐centers CC bond in π‐TCNE₂²⁻ dimers: Electrostatic or dispersion?

2006

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Abstract:The structure and stability of the -TCNE 2 2À dimers in K 2 TCNE 2 aggregates is revisited trying to find if the origin of their two-electron/four-centers CÀ ÀC bond are the electrostatic K þ -TCNE À interactions or the dispersion interactions between the anions. The study is done at the HF, B3LYP, CASSCF (2,2), and MCQDPT/CASSCF (2,2) levels using the 6-31þG(d) basis set. Our results show that the only minima of this aggregate that preserves the -TCNE 2 2À structure has the two K þ atoms placed in equatorial positions in between the two TCNE À planes. When the K þ atoms are placed along the D 2h axis of the anions the structure is not a minimum. The main energetic component responsible for the stability of these aggregates comes from the cation-anion interactions. However, a proper accounting of the dispersion component (as done in the MCQDPT/CASSCF (2,2) calculations) is needed to make the closedshell singlet more stable than the open-shell singlet. Thus, the bond results from the combination of the electrostatic and dispersion components, being the first the dominant one. The optimum geometry of the closed-shell singlet is very similar to the experimental one found in crystals.

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Section: Methodsmentioning

confidence: 99%

The origin of the two‐electron/four‐centers CC bond in π‐TCNE₂²⁻ dimers: Electrostatic or dispersion?

2006

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“…The exchange interaction was extracted from DFT calculations applying Noodleman's spin-projected broken-symmetry (BS) method. 44,45 The value for j DFT quoted above is obtained by considering only the spin contribution S Er = 3/2 of Er III . If the total Er angular momentum of J = L þ S = 15/2 is used, the smaller value of j DFT = À5.5 cm À1 is found.…”

Section: Articlementioning

confidence: 99%

Exchange Interaction of Strongly Anisotropic Tripodal Erbium Single-Ion Magnets with Metallic Surfaces

et al. 2014

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We present a comprehensive study of Er(trensal) single-ion magnets deposited in ultrahigh vacuum onto metallic surfaces. X-ray photoelectron spectroscopy reveals that the molecular structure is preserved after sublimation, and that the molecules are physisorbed on Au(111) while they are chemisorbed on a Ni thin film on Cu(100) single-crystalline surfaces. X-ray magnetic circular dichroism (XMCD) measurements performed on Au(111) samples covered with molecular monolayers held at temperatures down to 4 K suggest that the easy axes of the strongly anisotropic molecules are randomly oriented. Furthermore XMCD indicates a weak antiferromagnetic exchange coupling between the single-ion magnets and the ferromagnetic Ni/Cu(100) substrate. For the latter case, spin-Hamiltonian fits to the XMCD M(H) suggest a significant structural distortion of the molecules. Scanning tunneling microscopy reveals that the molecules are mobile on Au(111) at room temperature, whereas they are more strongly attached on Ni/Cu(100). X-ray photoelectron spectroscopy results provide evidence for the chemical bonding between Er(trensal) molecules and the Ni substrate. Density functional theory calculations support these findings and, in addition, reveal the most stable adsorption configuration on Ni/Cu(100) as well as the Ni–Er exchange path. Our study suggests that the magnetic moment of Er(trensal) can be stabilized via suppression of quantum tunneling of magnetization by exchange coupling to the Ni surface atoms. Moreover, it opens up pathways toward optical addressing of surface-deposited single-ion magnets.

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“…While this could certainly limit the application range of ECEPA, we must stress that spin-projection is routinely applied in Kohn-Sham density functional theory 36,37 (DFT) through Noodleman's approximation 38,39 or Yamaguchi's weighted-average approximation, 40,41 both of which remove spin-contamination only at first order.…”

Section: Introductionmentioning

confidence: 99%

Bridging Single- and Multireference Domains for Electron Correlation: Spin-Extended Coupled Electron Pair Approximation

Tsuchimochi

Ten‐no

2017

J. Chem. Theory Comput.

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We propose a size-consistent generalization of the recently developed spin-extended configuration interaction with singles and doubles (ECISD), where a CI wave function is explicitly spin-projected. The size-consistent effect is effectively incorporated by treating quadruples within the formulation of coupled electron pair approximation. As in coupled-cluster theory, quadruple excitations are approximated by a disconnected product of double excitations. Despite its conceptual similarity to the standard single-and multireference analogues, such a generalization requires careful derivation, as the spin-projected CI space is non-orthogonal and overcomplete. Although our methods generally yield better results than ECISD, size-consistency is only approximately retained because the action of a symmetry-projection operator is size-inconsistent. In this work, we focus on simple models where exclusion-principle-violating terms, which eliminate undesired contributions to the correlation effects, are either completely neglected or averaged. These models possess an orbital-invariant energy functional that is to be minimized by diagonalizing an energy-shifted effective Hamiltonian within the singles and doubles manifold. This allows for a straightforward generalization of the ECISD analytical gradients needed to determine molecular properties and geometric optimization. Given the multireference nature of the spin-projected Hartree-Fock method, the proposed approaches are expected to handle static correlation, unlike single-reference analogues. We critically assess the performance of our methods using dissociation curves of molecules, singlet-triplet splitting gaps, hyperfine coupling constants, and the chromium dimer. The size-consistency and size-extensivity of the methods are also discussed.

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Valence bond description of antiferromagnetic coupling in transition metal dimers

Cited by 2,454 publications

References 26 publications

The origin of the two‐electron/four‐centers CC bond in π‐TCNE₂²⁻ dimers: Electrostatic or dispersion?

The origin of the two‐electron/four‐centers CC bond in π‐TCNE₂²⁻ dimers: Electrostatic or dispersion?

Exchange Interaction of Strongly Anisotropic Tripodal Erbium Single-Ion Magnets with Metallic Surfaces

Bridging Single- and Multireference Domains for Electron Correlation: Spin-Extended Coupled Electron Pair Approximation

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Valence bond description of antiferromagnetic coupling in transition metal dimers

Cited by 2,454 publications

References 26 publications

The origin of the two‐electron/four‐centers CC bond in π‐TCNE22− dimers: Electrostatic or dispersion?

The origin of the two‐electron/four‐centers CC bond in π‐TCNE22− dimers: Electrostatic or dispersion?

Exchange Interaction of Strongly Anisotropic Tripodal Erbium Single-Ion Magnets with Metallic Surfaces

Bridging Single- and Multireference Domains for Electron Correlation: Spin-Extended Coupled Electron Pair Approximation

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The origin of the two‐electron/four‐centers CC bond in π‐TCNE₂²⁻ dimers: Electrostatic or dispersion?

The origin of the two‐electron/four‐centers CC bond in π‐TCNE₂²⁻ dimers: Electrostatic or dispersion?