Restricted Ensemble-Referenced Kohn−Sham versus Broken Symmetry Approaches in Density Functional Theory:  Magnetic Coupling in Cu Binuclear Complexes

Abstract

The performance of density functional theory in estimating the magnetic coupling constant in a series of Cu(II) binuclear complexes is investigated by making use of two open shell formalisms: the broken symmetry and the spin-restricted ensemble-referenced Kohn-Sham methods. The strong dependence of the calculated magnetic coupling constants with respect to the exchange-correlation functional is confirmed and found to be independent of whether spin symmetry is imposed or not. The use of a method which guarantees the spin state does not improve the correlation with the experiment and indeed shows some worsening due to an overestimation of the ferromagnetic interactions. However, with the present exchange-correlation functionals, a rather systematic deviation is found. Therefore, it would be possible to develop improved density functionals which will allow for a rigorous treatment of open shell systems in density functional theory.

Geometry and bonding in the ground and lowest triplet state of D6h symmetric crenellated edged C6[3m(m−1)+1]H6(2m−1) (m=2,…,6) graphene hydrocarbon molecules

“…The use of a single configuration, to represent the spin properties of open shell systems has been widely investigated in the recent years [34,35]. The DFT method can be used with confidence when there are other states in the vicinity of the ground state.…”

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“…This approach, known as unrestricted KS formalism is known to yield a qualitatively correct description of the bond breaking [153]. Alternatives to the broken spin-symmetry KS solution exist, including the multireference DFT [154], the local HF method [155], and the ensemble KS approach [156]. These new approaches, however, require development of new exchange-correlation functionals to obtain accurate numerical results.…”

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“…However, attempts to implement DFT-based method which preserves the spin symmetry have been made and applied to metallic binuclear complexes. [19][20][21] The other possibility is provided by post-Hartree-Fock ͑HF͒ methods. These are indeed computationally more expensive than DFT-based approaches, but have the advantage of a more solid conceptual basis and, beside the singlet-triplet energy gap, allow the computation of the eigenfunctions of the two spin states.…”