11 research outputs found
A Practical Guide to Surface Kinetic Monte Carlo Simulations
This review article is intended as a practical guide for newcomers to the
field of kinetic Monte Carlo (KMC) simulations, and specifically to lattice KMC
simulations as prevalently used for surface and interface applications. We will
provide worked out examples using the kmos code, where we highlight the central
approximations made in implementing a KMC model as well as possible pitfalls.
This includes the mapping of the problem onto a lattice and the derivation of
rate constant expressions for various elementary processes. Example KMC models
will be presented within the application areas surface diffusion, crystal
growth and heterogeneous catalysis, covering both transient and steady-state
kinetics as well as the preparation of various initial states of the system. We
highlight the sensitivity of KMC models to the elementary processes included,
as well as to possible errors in the rate constants. For catalysis models in
particular, a recurrent challenge is the occurrence of processes at very
different timescales, e.g. fast diffusion processes and slow chemical
reactions. We demonstrate how to overcome this timescale disparity problem
using recently developed acceleration algorithms. Finally, we will discuss how
to account for lateral interactions between the species adsorbed to the
lattice, which can play an important role in all application areas covered
here.Comment: This document is the final Author's version of a manuscript that has
been peer reviewed and accepted for publication in Frontiers in Chemistry. To
access the final edited and published work see
https://www.frontiersin.org/articles/10.3389/fchem.2019.00202/abstrac
Adsorption and reactivity of halogenated hydrocarbons on metal and semiconductor surfaces
We investigated the adsorption and reactivity of substituted hydrocarbons on Si and Cu surfaces using Grimmeâs vdWâcorrected DFT, CIâNEB and STM simulations. Halogenated hydrocarbons on surfaces are systems of particular interest. These molecules adsorb and selfâassembly at surfaces and many experimental works show that, if one provides energy to the complex, in the form of heat, light, or electrons dropped with an STM tip, they easily react resulting in single, or patterns of, chemisorbed atoms at specific and controllable sites. For instance, 1âchloropentane forms asymmetric (A) and symmetric (S) pairs on Si(001)â2Ă1. The rate of thermal reaction of A is greater than S in chlorinating room-temperature silicon. The energy threshold for electronâinduced reaction is also different. We have used DFT and NEB tools to explain the features of this system and we simulated STM images in agreement with the experiments. On the other hand, diiodobenzenes physisorbed on Cu(110) can act as molecular calipers. We have computationally modelled the adsorption of 1,3-diiodobenzene (mâDIB) on Cu(110) and simulated STM images for the four most stable configurations using the TersoffâHamann approach at different bias voltages. We find that all the adsorption orientations have comparable energy and we discuss the relative probabilities of experimental observation as well as the structural details. We have furthermore compared the electronic groundâstate reactivity of 1,3â and 1,4âdiiodobenzene in order to show that the different symmetry of the initial adsorbed state greatly affects reactivity. Since the studied systems provide a means to surface functionalization via siteâspecific imprinting of single atoms, we also propose a model for Cu nanoclusters on Cu(110) supported by one or two chemisorbed S (or Cl) atoms
Systematic Comparison of Genetic Algorithm and Basin Hopping Approaches to the Global Optimization of Si(111) Surface Reconstructions
We present a systematic study of two widely used material structure prediction methods, the Genetic Algorithm and Basin Hopping approaches to global optimization, in a search for the 3 Ă 3, 5 Ă 5, and 7 Ă 7 reconstructions of the Si(111) surface. The Si(111) 7 Ă 7 reconstruction is the largest and most complex surface reconstruction known, and finding it is a very exacting test for global optimization methods. In this paper, we introduce a modification to previous Genetic Algorithm work on structure search for periodic systems, to allow the efficient search for surface reconstructions, and present a rigorous study of the effect of the different parameters of the algorithm. We also perform a detailed comparison with the recently improved Basin Hopping algorithm using Delocalized Internal Coordinates. Both algorithms succeeded in either resolving the 3 Ă 3, 5 Ă 5, and 7 Ă 7 DAS surface reconstructions or getting âsufficiently closeâ, i.e., identifying structures that only differ for the positions of a few atoms as well as thermally accessible structures within kBT/unit area of the global minimum, with T = 300 K. Overall, the Genetic Algorithm is more robust with respect to parameter choice and in success rate, while the Basin Hopping method occasionally exhibits some advantages in speed of convergence. In line with previous studies, the results confirm that robustness, success, and speed of convergence of either approach are strongly influenced by how much the trial moves tend to preserve favorable bonding patterns once these appear
Global materials structure search with chemically motivated coordinates
Identification of relevant reaction pathways in ever more complex composite materials and nanostructures poses a central challenge to computational materials discovery. Efficient global structure search, tailored to identify chemically relevant intermediates, could provide the necessary first-principles atomistic insight to enable a rational process design. In this work we modify a common feature of global geometry optimization schemes by employing automatically generated collective curvilinear coordinates. The similarity of these coordinates to molecular vibrations enhances the generation of chemically meaningful trial structures for covalently bound systems. In the application to hydrogenated Si clusters, we concomitantly observe a significantly increased efficiency in identifying low-energy structures and exploit it for an extensive sampling of potential products of silicon-cluster soft landing on Si(001) surfaces
Global structure search for molecules on surfaces : efficient sampling with curvilinear coordinates
Efficient structure search is a major challenge in computational materials science. We present a modification of the basin hopping global geometry optimization approach that uses a curvilinear coordinate system to describe global trial moves. This approach has recently been shown to be efficient in structure determination of clusters [C. Panosetti et al., Nano Lett. 15, 8044â8048 (2015)] and is here extended for its application to covalent, complex molecules and large adsorbates on surfaces. The employed automatically constructed delocalized internal coordinates are similar to molecular vibrations, which enhances the generation of chemically meaningful trial structures. By introducing flexible constraints and local translation and rotation of independent geometrical subunits, we enable the use of this method for molecules adsorbed on surfaces and interfaces. For two test systems, trans-ÎČ-ionylideneacetic acid adsorbed on a Au(111) surface and methane adsorbed on a Ag(111) surface, we obtain superior performance of the method compared to standard optimization moves based on Cartesian coordinates
Learning to Use the Force: Fitting Repulsive Potentials in Density-Functional Tight-Binding with Gaussian Process Regression
The Density-Functional Tight Binding (DFTB) method is a popular semiempirical approximation to Density Functional Theory (DFT). In many cases, DFTB can provide comparable accuracy to DFT at a fraction of the cost, enabling simulations on length- and time-scales that are unfeasible with first principles DFT. At the same time (and in contrast to empirical interatomic potentials and force-fields), DFTB still offers direct access to electronic properties such as the band-structure. These advantages come at the cost of introducing empirical parameters to the method, leading to a reduced transferability compared to true first-principle approaches. Consequently, it would be very useful if the parameter-sets could be routinely adjusted for a given project. While fairly robust and transferable parameterization workflows exist for the electronic structure part of DFTB, the so-called repulsive potential Vrep poses a major challenge. In this paper we propose a machine-learning (ML) approach to fitting Vrep, using Gaussian Process Regression (GPR). The use of GPR circumvents the need for non-linear or global parameter optimization, while at the same time offering arbitrary flexibility in terms of the functional form. We also show that the proposed method can be applied to multiple elements at once, by fitting repulsive potentials for organic molecules containing carbon, hydrogen and oxygen. Overall, the new approach removes focus from the choice of functional form and parameterization procedure, in favour of a data-driven philosophy
Revisiting the storage capacity limit of graphite battery anodes: spontaneous lithium overintercalation at ambient pressure
The market quest for fast-charging, safe, long-lasting and performant
batteries drives the exploration of new energy storage materials, but also
promotes fundamental investigations of materials already widely used.
Presently, revamped interest in anode materials is observed -- primarily
graphite electrodes for lithium-ion batteries. Here, we focus on the upper
limit of lithium intercalation in the morphologically quasi-ideal highly
oriented pyrolytic graphite (HOPG), with a LiC stoichiometry corresponding
to 100\% state of charge (SOC). We prepared a sample by immersion in liquid
lithium at ambient pressure and investigated it by static Li nuclear
magnetic resonance (NMR). We resolved unexpected signatures of superdense
intercalation compounds, LiC. These have been ruled out for decades,
since the highest geometrically accessible composition, LiC, can only be
prepared under high pressure. We thus challenge the widespread notion that any
additional intercalation beyond LiC is not possible under ambient
conditions. We monitored the sample upon calendaric aging and employed ab
initio calculations to rationalise the NMR results. The computed relative
stabilities of different superdense configurations reveal that non-negligible
overintercalation does proceed spontaneously beyond the currently accepted
capacity limit