55 research outputs found
Revised self-consistent continuum solvation in electronic-structure calculations
The solvation model proposed by Fattebert and Gygi [Journal of Computational
Chemistry 23, 662 (2002)] and Scherlis et al. [Journal of Chemical Physics 124,
074103 (2006)] is reformulated, overcoming some of the numerical limitations
encountered and extending its range of applicability. We first recast the
problem in terms of induced polarization charges that act as a direct mapping
of the self-consistent continuum dielectric; this allows to define a functional
form for the dielectric that is well behaved both in the high-density region of
the nuclear charges and in the low-density region where the electronic
wavefunctions decay into the solvent. Second, we outline an iterative procedure
to solve the Poisson equation for the quantum fragment embedded in the solvent
that does not require multi-grid algorithms, is trivially parallel, and can be
applied to any Bravais crystallographic system. Last, we capture some of the
non-electrostatic or cavitation terms via a combined use of the quantum volume
and quantum surface [Physical Review Letters 94, 145501 (2005)] of the solute.
The resulting self-consistent continuum solvation (SCCS) model provides a very
effective and compact fit of computational and experimental data, whereby the
static dielectric constant of the solvent and one parameter allow to fit the
electrostatic energy provided by the PCM model with a mean absolute error of
0.3 kcal/mol on a set of 240 neutral solutes. Two parameters allow to fit
experimental solvation energies on the same set with a mean absolute error of
1.3 kcal/mol. A detailed analysis of these results, broken down along different
classes of chemical compounds, shows that several classes of organic compounds
display very high accuracy, with solvation energies in error of 0.3-0.4
kcal/mol, whereby larger discrepancies are mostly limited to self-dissociating
species and strong hydrogen-bond forming compounds.Comment: The following article has been accepted by The Journal of Chemical
Physics. After it is published, it will be found at
http://link.aip.org/link/?jcp
Electrostatics of solvated systems in periodic boundary conditions
Continuum solvation methods can provide an accurate and inexpensive embedding
of quantum simulations in liquid or complex dielectric environments.
Notwithstanding a long history and manifold applications to isolated systems in
open boundary conditions, their extension to materials simulations ---
typically entailing periodic-boundary conditions --- is very recent, and
special care is needed to address correctly the electrostatic terms. We discuss
here how periodic-boundary corrections developed for systems in vacuum should
be modified to take into account solvent effects, using as a general framework
the self-consistent continuum solvation model developed within plane-wave
density-functional theory [O. Andreussi et al. J. Chem. Phys. 136, 064102
(2012)]. A comprehensive discussion of real-space and reciprocal-space
corrective approaches is presented, together with an assessment of their
ability to remove electrostatic interactions between periodic replicas.
Numerical results for zero-dimensional and two-dimensional charged systems
highlight the effectiveness of the different suggestions, and underline the
importance of a proper treatement of electrostatic interactions in
first-principles studies of charged systems in solution
Transport properties of room temperature ionic liquids from classical molecular dynamics
Room Temperature Ionic Liquids (RTILs) have attracted much of the attention
of the scientific community in the past decade due the their novel and highly
customizable properties. Nonetheless their high viscosities pose serious
limitations to the use of RTILs in practical applications. To elucidate some of
the physical aspects behind transport properties of RTILs, extensive classical
molecular dynamics (MD) calculations are reported. Bulk viscosities and ionic
conductivities of butyl-methyl-imidazole based RTILs are presented over a wide
range of temperatures. The dependence of the properties of the liquids on
simulation parameters, e.g. system size effects and choice of the interaction
potential, is analyzed
Uncoupling System and Environment Simulation Cells for Fast-Scaling Modeling of Complex Continuum Embeddings
Continuum solvation models are becoming increasingly relevant in condensed
matter simulations, allowing to characterize materials interfaces in the
presence of wet electrified environments at a reduced computational cost with
respect to all atomistic simulations. However, some challenges with the
implementation of these models in plane-wave simulation packages still
persists, especially when the goal is to simulate complex and heterogeneous
environments. Among these challenges is the computational cost associated with
large heterogeneous environments, which in plane-wave simulations has a direct
effect on the basis-set size and, as a result, on the cost of the electronic
structure calculation. Moreover, the use of periodic simulation cells are not
well-suited for modeling systems embedded in semi-infinite media, which is
often the case in continuum solvation models. To address these challenges, we
present the implementation of a double-cell formalism, in which the simulation
cell used for the continuum environment is uncoupled from the one used for the
electronic-structure simulation of the quantum-mechanical system. This allows
for a larger simulation cell to be used for the environment, without
significantly increasing computational time. In this work, we show how the
double-cell formalism can be used as an effective PBC correction scheme for
non-periodic and partially periodic systems. The accuracy of the double-cell
formalism is tested using representative examples with different
dimensionalities, both in vacuum and in a continuum dielectric environment.
Fast convergence and good speedups are observed for all the simulation setups,
provided the quantum-mechanical simulation cell is chosen to completely fit the
electronic density of the system
Catalytic activity of Pt_(38) in the oxygen reduction reaction from first-principles simulations
The activity of truncated octahedral Pt_(38) clusters as a catalyst in the oxygen reduction reaction (ORR) is investigated via first-principles simulations. Three catalytic steps: O_2 dissociation (O_(2ads) → 2_O_(ads)), O hydration (O_(ads) + H_2O_(ads) → 2OH_(ads)), and H_2O formation (OH_(ads) + H_(ads) → H_2O_(ads)) are considered, in which all reactant species are co-adsorbed on the Pt_(38) cluster according to a Langmuir–Hinshelwood mechanism. The minimum structures and saddle points for these different steps are then calculated at the density-functional theory (DFT) level using a gradient-corrected exchange–correlation (xc-)functional and taking into account the effect of the solvent via a self-consistent continuum solvation model. Moreover, first-principles molecular dynamics (AIMD) simulations in which the H_2O solvent is explicitly described are performed to explore dynamic phenomena such as fast hydrogen transfer via meta-stable hydronium-type configurations and their possible role in ORR reaction paths. By comparing the present findings with previous results on the Pt(111) surface, it is shown that in such a nanometer-size cluster the rate-determining-step (rds) corresponds to H_2O formation, at variance with the extended surface in which O hydration was rate-determining, and that the overall reaction barrier is actually increased with respect to the extended system. This is in agreement with and rationalizes experimental results showing a decrease of ORR catalytic activity in the nanometer-size cluster range
Water on Pt(111): the importance of proton disorder
The structure of a water adlayer on Pt(111) surface is investigated by
extensive first principle calculations. Only allowing for proton disorder the
ground state energy can be found. This results from an interplay between
water/metal chemical bonding and the hydrogen bonding of the water network. The
resulting short O-Pt distance accounts for experimental evidences. The novelty
of these results shed a new light on relevant aspects of water-metal
interaction.Comment: 10 pages 4 figures (color
An In Situ Surface-Enhanced Infrared Absorption Spectroscopy Study of Electrochemical CO2 Reduction: Selectivity Dependence on Surface C-Bound and O-Bound Reaction Intermediates
The CO_{2} electro-reduction reaction (CORR) is a promising avenue to convert
greenhouse gases into high-value fuels and chemicals, in addition to being an
attractive method for storing intermittent renewable energy. Although
polycrystalline Cu surfaces have long known to be unique in their capabilities
of catalyzing the conversion of CO_{2} to higher-order C1 and C2 fuels, such as
hydrocarbons (CH_{4}, C_{2}H_{4} etc.) and alcohols (CH_{3}OH, C_{2}H_{5}OH),
product selectivity remains a challenge. In this study, we select three metal
catalysts (Pt, Au, Cu) and apply in situ surface enhanced infrared absorption
spectroscopy (SEIRAS) and ambient-pressure X-ray photoelectron spectroscopy
(APXPS), coupled to density-functional theory (DFT) calculations, to get
insight into the reaction pathway for the CORR. We present a comprehensive
reaction mechanism for the CORR, and show that the preferential reaction
pathway can be rationalized in terms of metal-carbon (M-C) and metal-oxygen
(M-O) affinity. We show that the final products are determined by the
configuration of the initial intermediates, C-bound and O-bound, which can be
obtained from CO_{2} and (H)CO_{3}, respectively. C1 hydrocarbons are produced
via OCH_{3, ad} intermediates obtained from O-bound CO_{3, ad} and require a
catalyst with relatively high affinity for O-bound intermediates. Additionally,
C2 hydrocarbon formation is suggested to result from the C-C coupling between
C-bound CO_{ad} and (H)CO_{ad}, which requires an optimal affinity for the
C-bound species, so that (H)CO_{ad} can be further reduced without poisoning
the catalyst surface. Our findings pave the way towards a design strategy for
CORR catalysts with improved selectivity, based on this
experimental/theoretical reaction mechanisms that have been identified
Kinetics and mechanism of the hydrogen peroxide reduction reaction on a graphite carbon ntride sensor
The kinetics and mechanism of the hydrogen peroxide reduction reaction (HPRR) on a recently proposed carbon-based electrode is studied by means of experiments and simulations. The electrode is highly oriented pyrolytic graphite (HOPG) modified by the deposition of a graphite carbon nitride (g-C3N4) film. Current transients obtained from chronoamperometry measurements allow us to propose a kinetic model for the HPRR on the surface. The model produces excellent fits of current transients, providing sensible rate constants for each electrocatalytic step. The rate constants obtained are consistent with low energy barriers for each step, suggesting outstanding electrocatalytic activity of the g-C3N4/HOPG electrode. Moreover, different trends are found for low and high analyte concentrations, evidencing a change in the reaction mechanism. To clarify the mechanisms involved in the reaction, first-principles atomistic simulations were performed. The different reaction steps were modeled at the substrate/water interface, including solvent environment through continuum embedding approaches. The simulated thermodynamics and kinetics of the different processes show that a significant role in the electrocatalytic activity of the system is associated with the geometrical rearrangements of the interface, with a critical role played by the corrugation/decorrugation processes of the outermost sheet of the electrode.Fil: Rojas, Mariana Isabel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - CĂłrdoba. Instituto de Investigaciones en FĂsico-quĂmica de CĂłrdoba. Universidad Nacional de CĂłrdoba. Facultad de Ciencias QuĂmicas. Instituto de Investigaciones en FĂsico-quĂmica de CĂłrdoba; ArgentinaFil: Andreussi, Oliviero. University of North Texas; Estados UnidosFil: Gomez, Cesar Gerardo. Universidad Nacional de CĂłrdoba. Instituto de InvestigaciĂłn y Desarrollo en IngenierĂa de Procesos y QuĂmica Aplicada. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - CĂłrdoba. Instituto de InvestigaciĂłn y Desarrollo en IngenierĂa de Procesos y QuĂmica Aplicada; ArgentinaFil: Avalle, Lucia Bernardita. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - CĂłrdoba. Instituto de FĂsica Enrique Gaviola. Universidad Nacional de CĂłrdoba. Instituto de FĂsica Enrique Gaviola; Argentin
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