Ligand spin polarization and antiferromagnetic coupling in transition metal dimers

Noodleman, Louis; Davidson, Ernest R.

doi:10.1016/0301-0104(86)80192-6

Cited by 955 publications

(626 citation statements)

References 18 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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“…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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“…[29][30][31][32][33][34][35][36] Moreover, the spin density-which serves as an additional fundamental quantity in the spin-DFT formalism commonly employed for open-shell systems-is qualitatively incorrect in some cases. [15,[37][38][39] To make things even worse, the treatment of low-spin states usually requires the use of a broken-symmetry description, [40][41][42][43] which provides an unphysical spin density by construction (see, e.g., Refs. [24,44] for a discussion).…”

Section: Introductionmentioning

confidence: 99%

Spin in density‐functional theory

Jacob¹,

Reiher²

2012

Int J of Quantum Chemistry

225

224

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The accurate description of open-shell molecules, in particular of transition metal complexes and clusters, is still an important challenge for quantum chemistry. Although density-functional theory (DFT) is widely applied in this area, the sometimes severe limitations of its currently available approximate realizations often preclude its application as a predictive theory. Here, we review the foundations of DFT applied to open-shell systems, both within the nonrelativistic and the relativistic framework. In particular, we provide an in-depth discussion of the exact theory, with a focus on the role of the spin density and possibilities for targeting specific spin states. It turns out that different options exist for setting up Kohn-Sham DFT schemes for open-shell systems, which imply different definitions of the exchangecorrelation energy functional and lead to different exact conditions on this functional. Finally, we suggest possible directions for future developments.

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Ligand spin polarization and antiferromagnetic coupling in transition metal dimers

Cited by 955 publications

References 18 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?

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

Spin in density‐functional theory

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Ligand spin polarization and antiferromagnetic coupling in transition metal dimers

Cited by 955 publications

References 18 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?

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

Spin in density‐functional theory

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Product

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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?