Although the optimized effective potential (OEP) method is a powerful method for going beyond the local density approximation of the density functional theory, it has not been widely used for extended systems. One of the reasons for this is that the OEP schemes used thus far contain an indefinite constant for extended systems that has to be determined in an ad hoc manner. In this paper, we propose a new practical scheme which enables us to determine the OEP without any such ambiguity. The principle is to determine the indefinite constant by imposing the requirement that the total energy be minimized. We apply the present scheme, in an exchange-only version, to crystalline alkali metals (Li, Na, and K), ferromagnetic Fe, and antiferromagnetic MnO.

We propose a framework for optimization of the chemical composition of multinary compounds with the aid of machine learning. The scheme is based on first-principles calculation using the Korringa-Kohn-Rostoker method and the coherent potential approximation (KKR-CPA). We introduce a method for integrating datasets to reduce systematic errors in a dataset, where the data are corrected using a smaller and more accurate dataset. We apply this method to values of the formation energy calculated by KKR-CPA for nonstoichiometric systems to improve them using a small dataset for stoichiometric systems obtained by the projector-augmented-wave (PAW) method. We apply our framework to optimization of RFe12-type magnet compounds (R1−αZα)(Fe 1−β Co β )12−γTiγ, and benchmark the efficiency in determination of the optimal choice of elements (R and Z) and ratio (α, β and γ) with respect to magnetization, Curie temperature and formation energy. We find that the optimization efficiency depends on descriptors significantly. The variable β, γ and the number of electrons from the R and Z elements per cell are important in improving the efficiency. When the descriptor is appropriately chosen, the Bayesian optimization becomes much more efficient than random sampling. * Descriptor Score True model arXiv:1903.09385v2 [cond-mat.mtrl-sci]

We theoretically study the structural stability of RFe 12 with the ThMn 12 structure (R: rare-earth element, La, Pr, Nd, Sm, Gd, Dy, Ho, Er, Tm, Lu, Y, or Sc, or group-IV element, Zr or Hf) based on density functional theory. The formation energy has a strong correlation with the atomic radius of R. The formation energy relative to simple substances decreases as the atomic radius decreases, except for R = Sc and Hf, while that relative to R 2 Fe 17 and bcc Fe has a minimum for R = Dy. The present results are consistent with recent experimental reports in which the partial substitution of Zr at R sites stabilizes RFe 12 -type compounds with R = Nd or Sm. Our results also suggest that the partial substitution of Y, Dy, Ho, Er, or Tm for Nd or Sm is a possible way to enhance the stability of the ThMn 12 structure. Under hydrostatic pressure, the formation enthalpy decreases up to ≈ 6 GPa and then starts to increase at higher pressures.