OCCUPATION NUMBERS IN DENSITY-FUNCTIONAL CALCULATIONS

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关键词: ENERGY;    SYSTEMS;   

DOI  :  10.1103/PhysRevB.52.10697

来源: SCIE

【 摘 要 】

It is the intention of this paper to rigorously clarify the role of occupation numbers in the current practical applications of the density-functional formalism. In these calculations one has to decide how to distribute a given, fixed number of electrons over a set of single-particle orbitals. The conventional choice is to have single-particle orbitals below the Fermi level completely occupied (with possible fractional occupations at the Fermi level) and the orbitals above the Fermi level empty. Although there is a certain confusion in the literature why this choice is superior to any others, the general belief is that it can be justified by treating the occupation numbers as variational parameters and then applying Janak's theorem or similar reasoning. We demonstrate that there is a serious flaw in those arguments, mainly the kinetic-energy functional and therefore the exchange-correlation functional are not differentiable with respect to the density for arbitrary occupation numbers. It is rigorously shown that in the present context of the density-functional calculations there is no freedom to vary occupation numbers. Occupation numbers cannot be considered as variational parameters.

【 授权许可】

Free