Quantum ESPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudo-potential and projector-augmented-wave approaches. Quantum ESPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement theirs ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.

The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements

We present an optimization algorithm to construct pseudopotentials and use it to generate a set of Optimized Norm-Conserving Vanderbilt (ONCV) pseudopotentials for elements up to Z=83 (Bi) (excluding Lanthanides). We introduce a quality function that assesses the agreement of a pseudopotential calculation with all-electron FLAPW results, and the necessary plane-wave energy cutoff. This quality function allows us to use a Nelder-Mead optimization algorithm on a training set of materials to optimize the input parameters of the pseudopotential construction for most of the periodic table. We control the accuracy of the resulting pseudopotentials on a test set of materials independent of the training set. We find that the automatically constructed pseudopotentials provide a good agreement with the all-electron results obtained using the FLEUR code with a plane-wave energy cutoff of approximately 60 Ry. arXiv:1502.00995v1 [cond-mat.mtrl-sci]

Halide perovskites constitute a new class of semiconductors that hold promise for low-cost solar cells and optoelectronics. One key property of these materials is the electron mobility, which determines the average electron speed due to a driving electric eld. Here we elucidate the atomic-scale mechanisms and theoretical limits of carrier mobilities in halide perovskites by performing a comparative analysis of the archetypal compound CH 3 NH 3 PbI 3 , its inorganic counterpart CsPbI 3 , and a classic semiconductor for LEDs, wurtzite GaN, using cutting-edge many-body ab initio calculations. We demonstrate that low-energy longitudinal-optical phonons associated with uctuations of the Pb-I bonds ultimately limit the mobility to 80 cm 2 /Vs at room temperature. By extending our analysis to a broad class of compounds, we identify a universal scaling law for the carrier mobility in halide perovskites, and we establish the design principles to realize high-mobility materials.

We elucidate the origin of the phonon-mediated superconductivity in 2H-NbS2 using the ab initio anisotropic Migdal-Eliashberg theory including Coulomb interactions. We demonstrate that superconductivity is associated with Fermi surface hot spots exhibiting an unusually strong electronphonon interaction. The electron-lattice coupling is dominated by low-energy anharmonic phonons, which place the system on the verge of a charge density wave instability. We also provide denitive evidence for two-gap superconductivity in 2H-NbS2, and show that the low-and high-energy peaks observed in tunneling spectra correspond to the Γ-and K-centered Fermi surface pockets, respectively. The present ndings call for further eorts to determine whether our proposed mechanism underpins superconductivity in the whole family of metallic transition metal dichalcogenides. PACS numbers: 74.70.Xa, 63.20.kd, 74.20.Fg, 74.25.Jb Transition metal dichalcogenides (TMDs) have generated considerable interest in recent years, since they provide an ideal playground for studying semiconductors, metals, and superconductors in two dimensions using the same structural template [13]. In the case of superconducting TMDs, one remarkable feature is that Cooper pair condensation usually coexists with a charge density wave (CDW) [4], raising the question on whether superconductivity and CDW co-operate or compete in these compounds [512].Within the family of superconducting TMDs, 2H-NbS 2 stands out as the only system for which a CDW phase has not been observed [13, 14]. This suggests that a comparative analysis of NbS 2 and other superconducting TMDs may help to clarify the interplay between the superconductive and the CDW instabilities in the entire family. 2H-NbS 2 is a phonon-mediated superconductor with a critical temperature T c = 5.7 K. Scanning tunneling spectroscopy (STS) measurements on this compound revealed two pronounced features in the density of states (DOS) at 0.53 meV and 0.97 meV below the critical temperature, providing strong indications of two-gap superconductivity [14]. However, so far microscopic calculations have considered only a single-gap scenario [15, 16].In this work we investigate the nature of the superconducting gap and the pairing mechanism in 2H-NbS 2 using the fully anisotropic ab initio Migdal-Eliashberg theory, and describe both electron-phonon and electronelectron interactions without any adjustable parameters. Our key nding is that a very signicant contribution to the superconducting pairing comes from the lowenergy anharmonic phonons with wavevectors near the line connecting the M and L points. These are the same phonons responsible for the CDW instability in other TMDs [8, 11, 1719], indicating that superconductivity in NbS 2 is intimately connected with a latent CDW. In agreement with the STS experiments of Ref. 14, we nd two distinct and anisotropic superconducting gaps.All calculations reported in this work were performed using density functional theory (DFT) in the local density approximation [20,21]. We employed...