Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set

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.

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QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

Multiwfn is a multifunctional program for wavefunction analysis. Its main functions are: (1) Calculating and visualizing real space function, such as electrostatic potential and electron localization function at point, in a line, in a plane or in a spatial scope. (2) Population analysis. (3) Bond order analysis. (4) Orbital composition analysis. (5) Plot density-of-states and spectrum. (6) Topology analysis for electron density. Some other useful utilities involved in quantum chemistry studies are also provided. The built-in graph module enables the results of wavefunction analysis to be plotted directly or exported to high-quality graphic file. The program interface is very user-friendly and suitable for both research and teaching purpose. The code of Multiwfn is substantially optimized and parallelized. Its efficiency is demonstrated to be significantly higher than related programs with the same functions. Five practical examples involving a wide variety of systems and analysis methods are given to illustrate the usefulness of Multiwfn. The program is free of charge and open-source. Its precompiled file and source codes are available from http://multiwfn.codeplex.com.

Multiwfn: a multifunctional wavefunction analyzer.

Recent extensions of the DMol3 local orbital density functional method for band structure calculations of insulating and metallic solids are described. Furthermore the method for calculating semilocal pseudopotential matrix elements and basis functions are detailed together with other unpublished parts of the methodology pertaining to gradient functionals and local orbital basis sets. The method is applied to calculations of the enthalpy of formation of a set of molecules and solids. We find that the present numerical localized basis sets yield improved results as compared to previous results for the same functionals. Enthalpies for the formation of H, N, O, F, Cl, and C, Si, S atoms from the thermodynamic reference states are calculated at the same level of theory. It is found that the performance in predicting molecular enthalpies of formation is markedly improved for the Perdew–Burke–Ernzerhof [Phys. Rev. Lett. 77, 3865 (1996)] functional.

From molecules to solids with the DMol3 approach

If they could be easily exfoliated, layered materials would become a diverse source of two-dimensional crystals whose properties would be useful in applications ranging from electronics to energy storage. We show that layered compounds such as MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2, NiTe2, BN, and Bi2Te3 can be efficiently dispersed in common solvents and can be deposited as individual flakes or formed into films. Electron microscopy strongly suggests that the material is exfoliated into individual layers. By blending this material with suspensions of other nanomaterials or polymer solutions, we can prepare hybrid dispersions or composites, which can be cast into films. We show that WS2 and MoS2 effectively reinforce polymers, whereas WS2/carbon nanotube hybrid films have high conductivity, leading to promising thermoelectric properties.

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Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials

We present the results of a crystal structure determination using neutron powder diffraction as well as the superconducting properties of the rare-earth sesquicarbide ${\mathrm{La}}_{2}{\mathrm{C}}_{3}$ $({T}_{c}\ensuremath{\approx}13.4\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ by means of specific-heat and upper critical field measurements. From the detailed analysis of the specific heat and a comparison with ab initio electronic structure calculations, a quantitative estimate of the electron-phonon coupling strength and the logarithmic average phonon frequency is made. The electron-phonon coupling constant is determined to be ${\ensuremath{\lambda}}_{ph}\ensuremath{\sim}1.35$. The electron-phonon coupling to low energy phonon modes is found to be the leading mechanism for superconductivity. Our results suggest that ${\mathrm{La}}_{2}{\mathrm{C}}_{3}$ is in the strong-coupling regime, and the relevant phonon modes are La related rather than C--C stretching modes. The upper critical field shows a clear enhancement with respect to the Werthamer-Helfand-Hohenberg prediction, consistent with the strong electron-phonon coupling. Possible effects on the superconducting properties due to the noncentrosymmetry of the crystal structure are discussed.

Strong electron-phonon coupling in the rare-earth carbide superconductor La 2 C 3

Experimental data for the anisotropy of the dielectric tensor {epsilon}={epsilon}{sub 1}+i{epsilon}{sub 2} in YBa{sub 2}Cu{sub 3}O{sub 6} are presented and interpreted. The spectra in the range between 1.7 and 5.2 eV were obtained by rotating-analyzer ellipsometry using a single crystal. It is found that the component of {epsilon}{sub 2} in the {ital ab} plane ({epsilon}{sub 2}{sup {ital a}}) displays a sharp peak at 4.1 eV, while the component along the {ital c} axis ({epsilon}{sub 2}{sup {ital c}}) has a less pronounced peak at 3.8 eV, a small peak at 4.2 eV, and a broad maximum near 5 eV. Similar structures were found in the interband spectra calculated using the local-density approximation. The 4-eV optical structure is caused by nearly parallel initial- and final-state bands whose wave-functions are located, respectively, in the O-Cu-O dumbbells and in the BaO-Cu-OBa triple layers. Intraionic 3{ital d}{r arrow}4{ital p} transitions in dumbbell Cu contribute 90% of the optical matrix elements.

Anisotropy of the dielectric function in YBa2Cu3O6.

Electron localization in solid-state structures of the elements : the elements : the diamond structure

We studied the metal-insulator transition in the $\mathrm{Ni}{\mathrm{S}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ system in $x=0$ and $x=0.5$ single crystals using angle-resolved photoemission. A narrow band, characteristic of many-body effects, develops near ${E}_{F}$ for $x=0.5$. This feature shifts away from ${E}_{F}$ at the transition temperature, a discontinuous change at the metalinsulator transition in a Mott-Hubbard system. We discuss our data in the context of recent theoretical calculations of the Hubbard model. The spectra support the general finding that a narrow peak of many-body nature develops at ${E}_{F}$ during the insulator-to-metal transition.

Electronic structure and the metal-insulator transition in NiS2-xSex.

count for the appearance of the superconducting phase in hcp Fe. However, the observed change in the transition temperature with increasing pressure is far too rapid compared with the calculated results. For comparison with the linear response results, we have computed the electron-phonon coupling also by using the rigid muffin-tin~RMT! approximation. From both the LR and the RMT results it appears that electron-phonon interaction alone cannot explain the small range of volume over which superconductivity is observed. It is shown that ferromagnetic/antiferromagnetic spin fluctuations ~SF! as well as scattering from magnetic impurities ~spin-ordered clusters! can account for the observed values of the transition temperatures but cannot substantially improve the agreement between the calculated and observed pressure/volume range of the superconducting phase. A simplified treatment of p-wave pairing leads to extremely small (<10 22 K! transition temperatures. Thus our calculations seem to rule out both s- and p-wave superconductivity in hcp Fe due to standard electron-phonon and SF interactions.

Ove Jepsen

Papers

Strong electron-phonon coupling in the rare-earth carbide superconductor La 2 C 3

Anisotropy of the dielectric function in YBa2Cu3O6.

Electron localization in solid-state structures of the elements : the elements : the diamond structure

Electronic structure and the metal-insulator transition in NiS2-xSex.

Pressure dependence of electron-phonon coupling and superconductivity in hcp Fe : A linear response study