Two exchange-correlation functionals compared for first-principles liquid water

The first-principles description of liquid water using ab initio molecular dynamics (AIMD) based on Density Functional theory (DFT) has recently been found to require long equilibration times, giving too low diffusivities and a clear over-structuring of the liquid. In the light of these findings we compare here the room-temperature description offered by two different exchange correlation functionals: BLYP, the most popular for liquid water so far, and RPBE, a revision of the widely used PBE. We find for RPBE a less structured liquid with radial distribution functions closer to the experimental ones than the ones of BLYP. The diffusivity obtained with RPBE for heavy water is still 20% lower than the corresponding experimental value, but it represents a substantial improvement on the BLYP value, one order of magnitude lower than experiment. These characteristics and the hydrogen-bond (HB) network imperfection point to an effective temperature ~3% lower than the actual simulation temperature for the RPBE liquid, as compared with BLYP's ~17% deviation. The too long O--O average nearest-neighbor distance observed points to an excessively weak HB, possibly compensating more fundamental errors in the DFT description.Comment: Jorunal reference adde

Direct Detection of sub-GeV Dark Matter with Semiconductor Targets

Dark matter in the sub-GeV mass range is a theoretically motivated but largely unexplored paradigm. Such light masses are out of reach for conventional nuclear recoil direct detection experiments, but may be detected through the small ionization signals caused by dark matter-electron scattering. Semiconductors are well-studied and are particularly promising target materials because their ${\cal O}(1~\rm{eV})$ band gaps allow for ionization signals from dark matter as light as a few hundred keV. Current direct detection technologies are being adapted for dark matter-electron scattering. In this paper, we provide the theoretical calculations for dark matter-electron scattering rate in semiconductors, overcoming several complications that stem from the many-body nature of the problem. We use density functional theory to numerically calculate the rates for dark matter-electron scattering in silicon and germanium, and estimate the sensitivity for upcoming experiments such as DAMIC and SuperCDMS. We find that the reach for these upcoming experiments has the potential to be orders of magnitude beyond current direct detection constraints and that sub-GeV dark matter has a sizable modulation signal. We also give the first direct detection limits on sub-GeV dark matter from its scattering off electrons in a semiconductor target (silicon) based on published results from DAMIC. We make available publicly our code, QEdark, with which we calculate our results. Our results can be used by experimental collaborations to calculate their own sensitivities based on their specific setup. The searches we propose will probe vast new regions of unexplored dark matter model and parameter space.Comment: 30 pages + 22 pages appendices/references, 17 figures, website at http://ddldm.physics.sunysb.edu/, v2 added references, minor edits to text and Figs. 2 and 14, version to appear in JHE

How Water's Properties Are Encoded in Its Molecular Structure and Energies.

How are water's material properties encoded within the structure of the water molecule? This is pertinent to understanding Earth's living systems, its materials, its geochemistry and geophysics, and a broad spectrum of its industrial chemistry. Water has distinctive liquid and solid properties: It is highly cohesive. It has volumetric anomalies-water's solid (ice) floats on its liquid; pressure can melt the solid rather than freezing the liquid; heating can shrink the liquid. It has more solid phases than other materials. Its supercooled liquid has divergent thermodynamic response functions. Its glassy state is neither fragile nor strong. Its component ions-hydroxide and protons-diffuse much faster than other ions. Aqueous solvation of ions or oils entails large entropies and heat capacities. We review how these properties are encoded within water's molecular structure and energies, as understood from theories, simulations, and experiments. Like simpler liquids, water molecules are nearly spherical and interact with each other through van der Waals forces. Unlike simpler liquids, water's orientation-dependent hydrogen bonding leads to open tetrahedral cage-like structuring that contributes to its remarkable volumetric and thermal properties

Fully ab-initio all-electron calculation of dark matter--electron scattering in crystals with evaluation of systematic uncertainties

We calculate target-material responses for dark matter--electron scattering at the \textit{ab-initio} all-electron level using atom-centered gaussian basis sets. The all-electron effects enhance the material response at high momentum transfers from dark matter to electrons, $q\gtrsim \mathcal{O}\left({10\ \alpha m_e}\right)$, compared to calculations using conventional plane wave methods, including those used in QEDark; this enhances the expected event rates at energy transfers $E \gtrsim 10$~eV, especially when scattering through heavy mediators. We carefully test a range of systematic uncertainties in the theory calculation, including those arising from the choice of basis set, exchange-correlation functional, number of unit cells in the Bloch sum, $\mathbf{k}$-mesh, and neglect of scatters with very high momentum transfers. We provide state-of-the-art crystal form factors, focusing on silicon and germanium. Our code and results are made publicly available as a new tool, called Quantum Chemistry Dark (``QCDark'').Comment: 15 pages, plus references and appendices, 12 figure

ECONOMIA CRIATIVA: DA DISCUSSÃO DO CONCEITO À FORMULAÇÃO DE POLÍTICAS PÚBLICAS

O artigo apresenta de maneira sintética o conceito de Economia Criativa e sua relação com a Economia da Inovação e do Conhecimento, além de destacar a importância das políticas públicas para o pleno desenvolvimento dos setores criativos. Trata-se de um conceito plural e ainda impreciso, a despeito dos inúmeros esforços de autores que buscaram definir suas fronteiras. É também um desafio teórico, na medida em que a economia mainstream tem dificuldades em lidar com atividades culturais, em especial as geradoras de bens intangíveis e que escapam à lógica da escassez. As políticas públicas de estímulo a estes setores têm se mostrado bem sucedidas na promoção do desenvolvimento, em especial na geração de emprego e renda em atividades em geral consideradas atraentes, sobretudo para os jovens. Mas persistem ainda lacunas na articulação das políticas de promoção da economia criativa com as demais, além da própria compreensão de seu potencial por parte do poder público.This article presents a brief outline of the concept of Creative Economy and its relation to the Economics of Innovation and Knowledge, highlighting the importance of public policies for the full development of the creative industries. It is a plural and still undefined concept, despite the numerous efforts of authors to define its borders. It is also a theoretical challenge to mainstream economics, for its difficulties in dealing with cultural activities, especially generators of intangible assets, and escape the logic of scarcity. Public policies to stimulate these sectors have been successful in promoting development, particularly in the generation of employment and income in activities considered attractive, especially for young people. But gaps still persist in articulating policies to promote creative economy with other economics, besides its own understanding of its potential by the government

Visualization of lithium-ion transport and phase evolution within and between manganese oxide nanorods.

Multiple lithium-ion transport pathways and local phase changes upon lithiation in silver hollandite are revealed via in situ microscopy including electron diffraction, imaging and spectroscopy, coupled with density functional theory and phase field calculations. We report unexpected inter-nanorod lithium-ion transport, where the reaction fronts and kinetics are maintained within the neighbouring nanorod. Notably, this is the first time-resolved visualization of lithium-ion transport within and between individual nanorods, where the impact of oxygen deficiencies is delineated. Initially, fast lithium-ion transport is observed along the long axis with small net volume change, resulting in two lithiated silver hollandite phases distinguishable by orthorhombic distortion. Subsequently, a slower reaction front is observed, with formation of polyphase lithiated silver hollandite and face-centred-cubic silver metal with substantial volume expansion. These results indicate lithium-ion transport is not confined within a single nanorod and may provide a paradigm shift for one-dimensional tunnelled materials, particularly towards achieving high-rate capability