162 research outputs found
Strengthening gold-gold bonds by complexing gold clusters with noble gases
We report an unexpectedly strong and complex chemical bonding of rare-gas
atoms to neutral gold clusters. The bonding features are consistently
reproduced at different levels of approximation within density-functional
theory and beyond: from GGA, through hybrid and double-hybrid functionals, up
to renormalized second-order perturbation theory. The main finding is that the
adsorption of Ar, Kr, and Xe reduces electron-electron repulsion within gold
dimer, causing strengthening of the Au-Au bond. Differently from the dimer, the
rare-gas adsorption effects on the gold trimer's geometry and vibrational
frequencies are mainly due to electron occupation of the trimer's lowest
unoccupied molecular orbital. For the trimer, the theoretical results are also
consistent with far-infrared multiple photon dissociation experiments.Comment: To be published in Inorganic Chemistry Communication
A quantum reactive scattering perspective on electronic nonadiabaticity
Based on quantum reactive-scattering theory, we propose a method for studying
the electronic nonadiabaticity in collision processes involving electron-ion
rearrangements. We investigate the state-to-state transition probability for
electron-ion rearrangements with two comparable approaches. In the first
approach the information of the electron is only contained in the ground-state
Born-Oppenheimer potential-energy surface, which is the starting point of
common reactive-scattering calculations. In the second approach, the electron
is explicitly taken into account and included in the calculations at the same
level as the ions. Hence, the deviation in the results between the two
approaches directly reflects the electronic nonadiabaticity during the
collision process. To illustrate the method, we apply it to the well-known
proton-transfer model of Shin and Metiu (one electron and three ions),
generalized by us in order to allow for reactive scattering channels. It is
shown that our explicit electron approach is able to capture electronic
nonadiabaticity and the renormalization of the reaction barrier near the
classical turning points of the potential in nuclear configuration space. In
contrast, system properties near the equilibrium geometry of the asymptotic
scattering channels are hardly affected by electronic nonadiabatic effects. We
also present an analytical expression for the transition amplitude of the
asymmetric proton-transfer model based on the direct evaluation of integrals
over the involved Airy functions.Comment: 14 page
Autocatalytic and cooperatively-stabilized dissociation of water on a stepped platinum surface
Water-metal interfaces are ubiquitous and play a key role in many chemical
processes, from catalysis to corrosion. Whereas water adlayers on atomically
flat transition metal surfaces have been investigated in depth, little is known
about the chemistry of water on stepped surfaces, commonly occurring in
realistic situations. Using first-principles simulations we study the
adsorption of water on a stepped platinum surface. We find that water adsorbs
preferentially at the step edge, forming linear clusters or chains, stabilized
by the cooperative effect of chemical bonds with the substrate and hydrogen
bonds. In contrast with flat Pt, at steps water molecules dissociate forming
mixed hydroxyl/water structures, through an autocatalytic mechanism promoted by
hydrogen bonding. Nuclear quantum effects contribute to stabilize partially
dissociated cluster and chains. Together with the recently demonstrated
attitude of water chains adsorbed on stepped Pt surfaces to transfer protons
via thermally activated hopping, these findings candidate these systems as
viable proton wires.Comment: 19 pages, 4 figure
Identifying outstanding transition-metal-alloy heterogeneous catalysts for the oxygen reduction and evolution reactions via subgroup discovery
In order to estimate the reactivity of a large number of potentially complex
heterogeneous catalysts while searching for novel and more efficient materials,
physical as well as data-centric models have been developed for a faster
evaluation of adsorption energies compared to first-principles calculations.
However, global models designed to describe as many materials as possible might
overlook the very few compounds that have the appropriate adsorption properties
to be suitable for a given catalytic process. Here, the subgroup-discovery
(SGD) local artificial-intelligence approach is used to identify the key
descriptive parameters and constrains on their values, the so-called SG rules,
which particularly describe transition-metal surfaces with outstanding
adsorption properties for the oxygen reduction and evolution reactions. We
start from a data set of 95 oxygen adsorption energy values evaluated by
density-functional-theory calculations for several monometallic surfaces along
with 16 atomic, bulk and surface properties as candidate descriptive
parameters. From this data set, SGD identifies constraints on the most relevant
parameters describing materials and adsorption sites that (i) result in O
adsorption energies within the Sabatier-optimal range required for the oxygen
reduction reaction and (ii) present the largest deviations from the linear
scaling relations between O and OH adsorption energies, which limit the
performance in the oxygen evolution reaction. The SG rules not only reflect the
local underlying physicochemical phenomena that result in the desired
adsorption properties but also guide the challenging design of alloy catalysts
The design of Kinetic Functionals for Many-Body Electron Systems : Combining analytical theory with Monte Carlo sampling of electronic configurations
In a previous work [L.Delle Site, J.Phys.A 40, 2787 (2007)] the derivation of
an analytic expression for the kinetic functional of a many-body electron
system has been proposed. Though analytical, the formula is still non local
(multidimensional) and thus not ideal for numerical applications. In this work,
by treating the test case of a uniform gas of interacting spinless electrons,
we propose a computational protocol which combines the previous analytic
results with the Monte Carlo (MC) sampling of electronic configurations in
space. This, we show, leads to an internally consistent scheme to design well
founded local kinetic functionals.Comment: 4 pages, 2 figure
SISSO: a compressed-sensing method for identifying the best low-dimensional descriptor in an immensity of offered candidates
The lack of reliable methods for identifying descriptors - the sets of
parameters capturing the underlying mechanisms of a materials property - is one
of the key factors hindering efficient materials development. Here, we propose
a systematic approach for discovering descriptors for materials properties,
within the framework of compressed-sensing based dimensionality reduction.
SISSO (sure independence screening and sparsifying operator) tackles immense
and correlated features spaces, and converges to the optimal solution from a
combination of features relevant to the materials' property of interest. In
addition, SISSO gives stable results also with small training sets. The
methodology is benchmarked with the quantitative prediction of the ground-state
enthalpies of octet binary materials (using ab initio data) and applied to the
showcase example of predicting the metal/insulator classification of binaries
(with experimental data). Accurate, predictive models are found in both cases.
For the metal-insulator classification model, the predictive capability are
tested beyond the training data: It rediscovers the available pressure-induced
insulator->metal transitions and it allows for the prediction of yet unknown
transition candidates, ripe for experimental validation. As a step forward with
respect to previous model-identification methods, SISSO can become an effective
tool for automatic materials development.Comment: 11 pages, 5 figures, in press in Phys. Rev. Material
Big Data of Materials Science - Critical Role of the Descriptor
Statistical learning of materials properties or functions so far starts with
a largely silent, non-challenged step: the choice of the set of descriptive
parameters (termed descriptor). However, when the scientific connection between
the descriptor and the actuating mechanisms is unclear, causality of the
learned descriptor-property relation is uncertain. Thus, trustful prediction of
new promising materials, identification of anomalies, and scientific
advancement are doubtful. We analyse this issue and define requirements for a
suited descriptor. For a classical example, the energy difference of
zincblende/wurtzite and rocksalt semiconductors, we demonstrate how a
meaningful descriptor can be found systematically.Comment: Accepted to Phys. Rev. Let
Theoretical evidence for unexpected O-rich phases at corners of MgO surfaces
Realistic oxide materials are often semiconductors, in particular at elevated
temperatures, and their surfaces contain undercoordiated atoms at structural
defects such as steps and corners. Using hybrid density-functional theory and
ab initio atomistic thermodynamics, we investigate the interplay of
bond-making, bond-breaking, and charge-carrier trapping at the corner defects
at the (100) surface of a p-doped MgO in thermodynamic equilibrium with an O2
atmosphere. We show that by manipulating the coordination of surface atoms one
can drastically change and even reverse the order of stability of reduced
versus oxidized surface sites.Comment: 5 papges, 4 figure
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