QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

We study the energetics, the electronic structure, and the ballistic transport of an infinite Au monatomic chain with an adsorbed CO molecule. We find that the bridge adsorption site is energetically favoured with respect to the atop site, both at the equilibrium Au-Au spacing of the chain and at larger spacings. Instead, a substitutional configuration requires a very elongated Au-Au bond, well above the rupture distance of the pristine Au chain. The electronic structure properties can be described by the Blyholder model, which involves the formation of bonding/antibonding pairs of 5σ and 2π ⋆ states through the hybridization between molecular levels of CO and metallic states of the chain. In the atop geometry, we find an almost vanishing conductance due to the 5σ antibonding states giving rise to a Fano-like destructive interference close to the Fermi energy. In the bridge geometry, instead, the same states are shifted to higher energies and the conductance reduction with respect to pristine Au chain is much smaller. We also examine the effects of strain on the ballistic transport, finding opposite behaviors for the atop and bridge conductances. Only the bridge geometry shows a strain dependence compatible with the experimental conductance traces.

We investigate the effect of epitaxial strain on the Mott metal-insulator transition (MIT) in perovskite systems with d 1 and d 2 electron configuration of the transition metal (TM) cation. We first discuss the general trends expected from the changes in the crystal-field splitting and in the hopping parameters that are induced by epitaxial strain. We argue that the strain-induced crystalfield splitting generally favors the Mott-insulating state, whereas the strain-induced changes in the hopping parameters favor the metallic state under compressive strain and the insulating state under tensile strain. Thus, the two effects can effectively cancel each other under compressive strain, while they usually cooperate under tensile strain, in this case favoring the insulating state. We then validate these general considerations by performing electronic structure calculations for several d 1 and d 2 perovskites, using a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT). We isolate the individual effects of strain-induced changes in either hopping or crystal-field by performing DMFT calculations where we fix one type of parameter to the corresponding unstrained DFT values. These calculations confirm our general considerations for SrVO3 (d 1 ) and LaVO3 (d 2 ), whereas the case of LaTiO3 (d 1 ) is distinctly different, due to the strong effect of the octahedral tilt distortion in the underlying perovskite crystal structure. Our results demonstrate the possibility to tune the electronic properties of correlated TM oxides by using epitaxial strain, which allows to control the strength of electronic correlations and the vicinity to the Mott MIT. arXiv:1608.03523v1 [cond-mat.str-el]