2,843 research outputs found
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
Unexpected differences between thermal and photoinitiated cationic curing of a diglycidyl ether of bisphenol A modified with a multiarm star poly(styrene)-b-poly(ε-caprolactone) polymer
The effect of adding a multiarm star poly(styrene)-b-poly(ε-caprolactone) polymer on the cationic thermal and photoinitiated curing of diglycidyl ether of bisphenol A was studied. This star-polymer decelerated the thermal curing of diglycidyl ether of bisphenol A and modified the final structure of the epoxy matrix. The photocuring was influenced significantly by the addition of the multiarm star. When the proportion of this modifier added was 5%, much more time was necessary for complete photocuring (160 min at 40ºC). In the presence of 10% of modifier, the degree of photocuring reached was very low (0.196 at 120°C). A subsequent thermal post-curing was necessary to cure completely the system. During photocuring in presence of poly(styrene)-b-poly(ε-caprolactone), the formation of dormant species, which are reactivated when the temperature increases, takes places. The kinetics of the thermal curing and the photocuring was analyzed using an isoconversional method due to the complexity of the reactive process. Applying this method, it has been confirmed the dependence of activation energy on the degree of conversion. The fracture morphology analyzed by scanning electron microscopy exhibited a second phase originated during photocuring by the presence of the modifier
Nonsingular potentials from excited state factorization of a quantum system with position dependent mass
The modified factorization technique of a quantum system characterized by
position-dependent mass Hamiltonian is presented. It has been shown that the
singular superpotential defined in terms of a mass function and a excited state
wave function of a given position-dependent mass Hamiltonian can be used to
construct non-singular isospectral Hamiltonians. The method has been
illustrated with the help of a few examples.Comment: Improved version accepted in J. Phys.
Anomalous Nuclear Quantum Effects in Ice
One striking anomaly of water ice has been largely neglected and never
explained. Replacing hydrogen (H) by deuterium (H) causes ice to
expand, whereas the "normal" isotope effect is volume contraction with
increased mass. Furthermore, the anomaly increases with temperature , even
though a normal isotope shift should decrease with and vanish when is
high enough to use classical nuclear motions. In this study, we show that these
effects are very well described by {\it ab initio} density functional theory.
Our theoretical modeling explains these anomalies, and allows us to predict and
to experimentally confirm a counter effect, namely that replacement of O
by O causes a normal lattice contraction.Comment: 5 pages, 3 figure
Enhancing structure relaxations for first-principles codes: an approximate Hessian approach
We present a method for improving the speed of geometry relaxation by using a
harmonic approximation for the interaction potential between nearest neighbor
atoms to construct an initial Hessian estimate. The model is quite robust, and
yields approximately a 30% or better reduction in the number of calculations
compared to an optimized diagonal initialization. Convergence with this
initializer approaches the speed of a converged BFGS Hessian, therefore it is
close to the best that can be achieved. Hessian preconditioning is discussed,
and it is found that a compromise between an average condition number and a
narrow distribution in eigenvalues produces the best optimization.Comment: 9 pages, 3 figures, added references, expanded optimization sectio
Dual-curable stereolithography resins for superior thermomechanical properties
Stereolithography (SL) stands out as a relatively fast additive manufacturing method to produce thermoset components with high resolutions. The majority of SL resins consist of acrylate monomers which result in materials with cur-ing-induced shrinkage problems and this, in addition to the incomplete and non-uniform conversions reached in the SL process, results in poor mechanical properties. To address this issue, a dual-curing formulation was developed by mixing an epoxy monomer into a commercial multi-acrylate SL resin: the first curing stage is acrylate free-radical photopolymerization at ambient temperature, and the second curing stage is cationic epoxy homopolymerization at higher temperatures. The fully dual-cured materials are macroscopically homogeneous, with nanoscale domains observed by Atomic Force Mi-croscopy (AFM), and with unimodal tan delta peaks observed in Dynamic Mechanical Analysis (DMA). The uncured material was storage stable at ambient conditions for at least 9 weeks since the epoxy part was virtually unreactive at these temper-atures. With the dual-cured materials, a nearly 10-fold increase in Young’s modulus was achieved over the neat acrylate resin. At the thermal curing stage, the presence of diperoxyketal thermal radical initiator to the liquid formulation facilitated the polymerization of unreacted acrylates that remained from the SL process simultaneously with epoxy homopolymerization and helped the material attain improved properties
Coherent state of a nonlinear oscillator and its revival dynamics
The coherent state of a nonlinear oscillator having a nonlinear spectrum is
constructed using Gazeau Klauder formalism. The weighting distribution and the
Mandel parameter are studied. Details of the revival structure arising from
different time scales underlying the quadratic energy spectrum are investigated
by the phase analysis of the autocorrelation function
TecnologÃas del hidrógeno
El interés por las tecnologÃas del hidrógeno ha crecido en los últimos años, principalmente porque una economÃa basada en el hidrógeno puede dar respuesta a los grandes desafÃos de la economÃa global del futuro: seguridad energética y cambio climático. Aprovechando este impulso, cada vez son más los paÃses que están implementando un número creciente de polÃticas en favor del hidrógeno. Prueba de ello es la Estrategia Europea del Hidrógeno que establece al hidrógeno como un elemento esencial en| la descarbonización total del actual sistema energético para alcanzar el compromiso de la UE con la neutralidad de carbono en 2050. No obstante, el desarrollo exitoso de las tecnologÃas del hidrógeno requiere que todos los actores, incluidos los sectores público y privado, aumenten sus esfuerzos para acelerar su despliegue y hacer que su implantación a gran escala resulte competitiva. Los grupos de investigación que forman parte del área de trabajo de tecnologÃas del hidrógeno, dentro de la Plataforma Temática Interdisciplinar PTI Mobility 2030 del CSIC, trabajan en este sentido, desarrollando su labor en áreas tan diversas como la generación, el almacenamiento, la distribución y los usos del hidrógeno.
The interest in hydrogen technologies has grown in recent years, mainly because an economy based on hydrogen can help to solve important challenges related to the global economy of the future: energy security and climate change. Taking advantage of this momentum, more and more countries are implementing a growing number of policies related to hydrogen. Indeed, the European Hydrogen Strategy establishes hydrogen as essential drivers for the total decarbonization of the current energy system in order to achieve the Ells commitment related to carbon neutrality by 2050. However, the successful development of the hydrogen technologies requires the collaboration of the public and private sectors to accelerate its deployment and make more competitive its implementation at large-scale. The research groups that take part of the line of work dedicated to hydrogen technologies, within the CSIC Interdisciplinary Thematic Platform PTI Mobility 2030, work in this regard, developing their investigations in several important areas related to the hydrogen technologies such as hydrogen generation, storage, distribution and uses
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