Based on the work of Gritsenko et al. (GLLB) [Phys. Rev. A 51, 1944], the method of Kuisma et al. [Phys. Rev. B 82, 115106 (2010)] to calculate the band gap in solids was shown to be much more accurate than the common local density approximation (LDA) and generalized gradient approximation (GGA). The main feature of the GLLB-SC potential (SC stands for solid and correlation) is to lead to a nonzero derivative discontinuity that can be conveniently calculated and then added to the Kohn-Sham band gap for a comparison with the experimental band gap. In this work, a thorough comparison of GLLB-SC with other methods, e.g., the modified Becke-Johnson (mBJ) potential [F. Tran and P. Blaha, Phys. Rev. Lett. 102, 226401 (2009)], for electronic, magnetic, and density-related properties is presented. It is shown that for the band gap, GLLB-SC does not perform as well as mBJ for systems with a small band gap and strongly correlated systems, but is on average of similar accuracy as hybrid functionals. The results on itinerant metals indicate that GLLB-SC overestimates significantly the magnetic moment (much more than mBJ does), but leads to excellent results for the electric field gradient, for which mBJ is in general not recommended. In the aim of improving the results, variants of the GLLB-SC potential are also tested.

Standard Landau theory coupled to infinitesimal strain allows a concise description of the temperature-driven ferroelectric tetragonal-to-cubic phase transition in PbTiO 3 at ambient pressure. Unfortunately, it fails to cover its high-pressure counterpart at ambient temperature. For example, the experimental transition pressure is vastly underestimated, and neither the change from first to second order with increasing pressure nor the unusual pressure dependence of the tetragonal unit cell parameters observed in experiment are reproduced. Here we demonstrate that a combination of density functional theory and a recently constructed finite-strain extension of Landau theory provides a natural mechanism for resolving these discrepancies between theory and experiment. Our approach also allows us to determine the full tetragonal-cubic phase boundary in the (P ,T) plane including an estimate of the tricritical point. We show that a careful analysis of the thermal elastic baseline is an essential ingredient to the success of this theory.

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The structural, elastic, vibrational, and electronic properties of RbCaF 3 in the cubic and low-temperature tetragonal phases have been studied at the ab initio level with density functional theory. Using various exchange-correlation functionals of the generalized gradient approximation for structural properties like the CaF 6 octahedron rotational angle or ratio c/a of the tetragonal lattice constants, it is found that the best agreement with experiment is obtained with the PBEsol and Wu and Cohen (WC) functionals. The fundamental band gap is calculated to be direct and indirect in the tetragonal and cubic phases, respectively. The relation between the cubic and tetragonal phases is studied by monitoring the cubic zone boundary soft mode phonon R 15 as well as the c/a ratio and tilt angle. The results for the Born effective charge tensors are also reported in order to study the effect of the long-range Coulomb interactions. We also investigated the corresponding pressure driven phase transition at T = 0 K, which we observe to be of second order in contrast to the first-order character experimentally detected for the structurally similar high pressure transition at ambient temperature. Based on recently developed finite strain Landau theory, we offer a possible explanation for this peculiar change of character.

In recent years, finite strain Landau theory has been gradually developed as both a conceptual as well as a quantitative framework to study high pressure phase transitions of the group-subgroup type. In the current paper, we introduce a new version of this approach which is based on symmetry-adapted finite strains. This results in a substantial simplification of the original formulation. Moreover, it allows for replacing the clumsy use of truncated Taylor expansions by a convenient functional parametrization. Both the weaknesses of the traditional Landau approach based on infinitesimal strains as well as the major improvements made possible by our new parametrization are illustrated in great detail in an application to the ambient temperature high pressure transition of the perovskite KMnF 3 .