XVIth International Workshop on
Quantum Systems in
Chemistry and Physics
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Origin of Antiferromagnetism in Molecular and Periodic Systems
in Original Kohn-Sham Local Density Approximation
Kimichika FUKUSHIMA
Department of Advanced Reactor System Engineering
Toshiba Nuclear Engineering Service Corporation
8, Shinsugita-cho, Isogo-ku, Yokohama, 235-8523, Japan
This study presents a solution to the issue, which attracted attention due to the discovery of copper (Cu) oxides in 1986, of whether LDA (local density approximation) can describe antiferromagnetism. From an early stage, many LDA band structure calculations failed to reproduce the insulating antiferromagnetic state. The Hubbard model predicts antiferromagnetism in a system for appropriate conditions. The authorís LDA calculations were performed for elongated hydrogen molecules comprising multiple atoms using the discrete variational (DV) molecular orbital method. The LDA employed is the original Kohn-Sham formalism, since the magnetic properties by GGA (generalized gradient approximation) are similar to the original Kohn-Sham results than those by the VWN (Vosko-Wilk-Nusair) approximation. The DV method with numerically calculated basis atomic orbitals derived antiferromagnetic ordering for hydrogen molecules at long interatomic separations. The DV method for Cu oxide molecules looked as if the method could not describe antiferromagnetism, where a well potential with a usual depth of about -1 Hartree within an ionic radius was added solely to the potential for generating O2- basis atomic orbitals. However, the author finally arrived at the antiferromagnetism description via a reduced well potential depth after long parameter surveys [1,2]. The calculation was generalized to a periodic system CaCuO2 using a method employing the Bloch type linear combination of atomic orbitals with all electrons [3]. Furthermore, we determined a spherically averaged well potential depth having originated from the Coulomb potential by the nucleus and electron clouds around O2- in a solid. The system revealed the antiferromagnetic ordering due to a shallow well depth. Since the Coulomb potential induced well for the anionic basis set is general, this method is applied to molecular orbital calculations.

References
[1] K. Fukushima, J. Phys. Soc. Jpn. 69 (2000) 1247.
[2] K. Fukushima, Adv. Quant. Chem. 54 (2008) 47.
[3] K. Fukushima, Int. J. Quant. Chem. (2011) DOI: 10.1002/qua.23146.



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