Long-timescale simulations of H2_2O admolecule diffusion on Ice Ih(0001) surfaces

Long-timescale simulations of the diffusion of a H$_2$O admolecule on the (0001) basal plane of ice Ih were carried out over a temperature range of 100 to 200 K using the adaptive kinetic Monte Carlo method and TIP4P/2005f interaction potential function. The arrangement of dangling H atoms was varied from the proton-disordered surface to the perfectly ordered Fletcher surface. A large variety of sites was found leading to a broad distribution in adsorption energy at both types of surfaces. Up to 4 % of the sites on the proton-disordered surface have an adsorption energy exceeding the cohesive energy of ice Ih. The mean squared displacement of a simulated trajectory at 175 K for the proton-disordered surface gave a diffusion constant of 6$\cdot$10$^{-10}$ cm$^2$/s, consistent with an upper bound previously reported from experimental measurements. During the simulation, dangling H atoms were found to rearrange so as to reduce clustering, thereby approaching a linear Fletcher type arrangement. Diffusion on the perfectly ordered Fletcher surface was estimated to be significantly faster, especially in the direction along the rows of dangling hydrogen atoms. From simulations over the range in temperature, an effective activation energy of diffusion was estimated to be 0.16 eV and 0.22 eV for diffusion parallel and perpendicular to the rows, respectively. Even a slight disruption of the rows of the Fletcher surface made the diffusion isotropic.Comment: 24 pages, 8 figures, 1 tabl

Design of two-dimensional particle assemblies using isotropic pair interactions with an attractive well

Using ground-state and relative-entropy based inverse design strategies, isotropic interactions with an attractive well are determined to stabilize and promote as- sembly of particles into two-dimensional square, honeycomb, and kagome lattices. The design rules inferred from these results are discussed and validated in the dis- covery of interactions that favor assembly of the highly open truncated-square and truncated-hexagonal lattices.Comment: 11 pages, 5 figures and supplemental materia

Theoretical studies of Van der Waals clusters

The vibrational energy levels of various rare gas trimers, Ar(_3), Ne(_3), He(_3), Ar(_2)Ne and Ne(_2)Ar, have been calculated using a coupled channel approach. We have compared results obtained with previous calculations. The existence of Efi-mov states in He(_3) has been investigated, and no evidence of their existence has been found. The affect of the Eckart conditions on embedding axis into a rotating-vibrating system has been investigated for several rare gas systems. A wide range of rare gas trimers have been studied, Ar(_3), He(_2)Ar, Ar(_2)He, Ar(_2)Ne and Ne(_2)Ar. For each trimer the full range of molecular motion is investigated. The low energy minima for the Ar(_n)N(_2) and Ne(_n)N(_2) systems have been found using simulated annealing search, and a gradient based minimisation technique, of a pairwise potential energy surface. Clusters with n ≥ 12 have been studied, and first solvation shells for both systems have been proposed. For each value of n, for n = 1 - 12, the first few low energy minima of the potential energy surface have been found. From these studies, we have gained a detailed understanding of the interplay of forces that determine the low energy structures for these systems. The affect of three-body interactions on the low energy minima both rare gas-N(_2) systems has been studied. In both system, rare gas-rare gas and rare gas- threebody interactions have been taken into account. This study has shown that the three-body forces have a small affect on the low energy structures of each system

A comparative study of model ingredients: fragmentation in heavy-ion collisions using quantum molecular dynamics model

We aim to understand the role of NN cross-sections, equation of state as well as different model ingredients such as width of Gaussian, clusterisation range and different clusterisation algorithms in multifragmentation using quantum molecular dynamics model. We notice that all model ingredients have sizable effect on the fragment pattern.Comment: 12 Pages, 4 Figure

Modeling and numerical study of the diffusion of point defects in α−iron

Le fer et les alliages à base de fer présentent un intérêt considérable pour la communauté de la modélisation des matériaux en raison de l’immense importance technologique de l’acier. Les alliages ferritiques à base de fer sont largement utilisés dans les industries aéronautique et nucléaire en raison de leur résistance mécanique élevée, de leur faible dilatation à haute température et de leur résistance à la corrosion. Ces propriétés sont cependant affectées par des défauts ponctuels intrinsèques et extrinsèques. Dans cette thèse, nous décrivons en détail la cinétique des défauts ponctuels dans le fer α en utilisant la technique d’activation-relaxation cinétique (ARTc), une méthode de Monte Carlo cinétique hors réseau avec construction de catalogue à la volée. Plus précisément, nous nous intéressons aux mécanismes de diffusion du carbone (C) et des amas de lacunes dans le fer α. Dans un premier temps, nous étudions l’effet de la pression sur la diffusion du carbone dans le joint de grains de fer α. Nous constatons que l’effet de la pression peut fortement modifier la stabilité et la diffusivité du carbon dans le joint de grains d’une manière qui dépend étroitement de l’environnement local et de la nature de la déformation. Ceci peut avoir un impact majeur sur l’évolution des matériaux hétérogènes, avec des variations de pression locale qui altéreraient fortement la diffusion à travers le matériau. Nous étudions également l’évolution structurale des amas de lacunes contenant de deux à huit lacunes dans le fer α. Nous décrivons en détail le paysage énergétique, la cinétique globale et les mécanismes de diffusion associés à ces défauts. Nos résultats montrent des mécanismes de diffusion complexes même pour des défauts aussi simples que de petits amas de lacunes. Enfin, dans le dernier chapitre, nous discutons une approche de gestion de petites barrières par bassin local dans ARTc. Les simulations de Monte Carlo cinétiques deviennent inefficaces dans les systèmes où le paysage énergétique est constitué de bassins avec de nombreux états reliés par des barrières énergétiques très faibles par rapport à celles nécessaires pour quitter ces bassins. Au fur et à mesure que le système évolue état par état, il est beaucoup plus susceptible d’effectuer des événements répétés (appelés oscillateurs) à l’intérieur du bassin d’énergie de piégeage que de s’échapper du bassin. De tels osccilateurs ne font pas progresser la simulation et ne fournissent que peu d’informations au-delà d’uen première évaluation de ces états. Notre algorithme de bassin local détecte, à la volée, des groupes d’états oscillants et les consolide en bassins locaux, que nous traitons avec la méthode de taux moyen d’auto-construction de bassin (bac-MRM), une approche de type équation maîtresse selon la méthode du taux moyen.Iron and iron-based alloys are of considerable interest to the materials modelling community because of the immense technological importance of steel. Iron-based ferritic alloys are widely used in aeronautic and nuclear industries due to their high mechanical strength, low expansion at high temperatures, and corrosion resistance. These properties are affected by intrinsic and extrinsic point defects, however. In this thesis, we describe in detail the kinetics of point defects in α−iron using the kinetic activation-relaxation technique (kART), an off-lattice kinetic Monte Carlo method with on-the-fly catalog building. More specifically, we focus on the diffusion mechanisms of carbon and vacancy clusters in α−iron. First, we study the pressure effect on carbon diffusion in the grain boundary (GB) of α−iron. We find that the effect of pressure can strongly modify the C stability and diffusivity in the GB in ways that depend closely on the local environment and the nature of the deformation. This can have a major impact on the evolution of heterogeneous materials, with variations of local pressure that would strongly alter diffusion across the material. We also study the structural evolution of vacancy clusters containing two to eight vacancies in α−iron. We describe in detail the energy landscape, overall kinetics, and diffusion mechanisms associated with these defects. Our results show complex scattering mechanisms even for defects as simple as small vacancy clusters. Finally, in the last chapter, we discuss a local basin approach to managing low barrier events in the kART. Kinetic Monte Carlo simulations become inefficient in systems where the energy landscape consists of basins with numerous states connected by very low energy barriers compared to those needed to leave these basins. As the system evolves state by state, it is much more likely to perform repeated events (so-called flickers) inside the trapping energy basin than to escape the basin. Such flickers do not progress the simulation and provide little insight beyond the first identification of those states. Our local basin algorithm detects, on the fly, groups of flickering states and consolidates them into local basins, which we treat with the basin-auto-constructing Mean Rate Method (bac-MRM), a master equation-like approach based on the mean-rate method

An Infeasible Point Method for Minimizing the Lennard-Jones Potential

Minimizing the Lennard-Jones potential, the most-studied modelproblem for molecular conformation, is an unconstrained globaloptimization problem with a large number of local minima. In thispaper, the problem is reformulated as an equality constrainednonlinear programming problem with only linear constraints. Thisformulation allows the solution to approached through infeasibleconfigurations, increasing the basin of attraction of the globalsolution. In this way the likelihood of finding a global minimizeris increased. An algorithm for solving this nonlinear program isdiscussed, and results of numerical tests are presented.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44788/1/10589_2004_Article_140555.pd

Feet on the potential energy surface, head in the π clouds

The landscape of a potential energy surface is marked by chemically interesting features. Hills and valleys correspond to transition states and reactive intermediates; the deepest valley gives the most stable configuration. Mapping these features for individual molecules and for the interactions between molecules is one of the goals of computational chemistry. The dispersion energy is a weak attractive force in intermolecular interactions. Dispersion energy results from a purely quantum mechanical effect, in which instantaneous multipoles on one molecule induce multipoles on another. Among neutral atoms or molecules that lack permanent multipole moments, the dispersion interaction is the principal attractive force. Dispersion also plays a significant role in the interaction between molecules with diffuse π clouds. This interaction is often difficult to capture with standard computational chemistry methods, so a comparison of the results obtained with various methods is itself important. This work presents explorations of the potential energy surface of clusters of atoms and of the interactions between molecules. First, structures of small aluminum clusters are examined and classified as ground states, transition states, or higher-order saddle points. Subsequently, the focus shifts to dispersion-dominated π-π interactions when the potential energy surfaces of benzene, substituted benzene, and pyridine dimers are explored. Because DNA nucleotide bases can be thought of as substituted heterocycles, a natural extension of the substituted benzene and pyridine investigations is to model paired nucleotide bases. Finally, the success of the dispersion studies inspires the development of an extension to the computational method used, which will enable the dispersion energy to be modeled - and the potential energy surface explored - in additional chemical systems