Bridging-Time Scale Techniques and their Applications in Atomistic Computational Science

Computation of the Coulomb sums is usually the most time-consuming part in the molecular-dynamic simulation of the long-range-interacting systems, such as the strongly-coupled plasmas. Here, we are going to discuss a new method based on using Green function in Fourier representation on a multi-dimensional torus. It was derived by exploiting the recent mathematical results from the theory of Pontryagin invariants and Milnor's linking numbers [D. DeTurck, et al. arXiv:1101.3374]. As follows from our first numerical tests, the new algorithm provides a good convergence at large interparticle distances as well as avoids the problem of "close encounters" at small distances.

Using density-functional method together with Green’s function techniques, we studied the effects of structural transformations on electronic structure and transport properties of layered tin dichalcogenides. Our band structure calculations showed that all bulk structures are indirect-gap semiconductors. The band gap values increase from bulk to monolayers due to the quantum confinement and become direct in the case of SnS and SnSe. The average effective electron mass is lower for SnS(Se) in comparison to SnS2(Se2) and Sn2S3(Se3) suggesting high electron mobility in this material. Our quantum transport calculations indicate strong anisotropy in the electron conductance through Sn2S3(Se3), while SnS2(Se2) has almost isotropic in-plane conductivity. The anisotropic conductivity can originate from the anisotropic atomic arrangement, which leads to different transmission pathways in the material. These findings could be helpful for the use of tin dichalcogenides in high performance electronic devices.

Preferred migration pathways of oxygen anion vacancies in yttria-stabilized zirconia (YSZ) is investigated. Activation barriers calculated using density functional theory calculations of [1] are incorporated in a kinetic Monte Carlo (KMC) model to calculate the ionic conductivity. A maximum in the ionic conductivity with increasing Y^{3+} concentration is explained in terms of a percolation network formed by the introduction of the dopant atoms. The structure of the network and the associated migration rates determines the ionic conductivity. The mechanism of vacancy diffusion in yttria stabilized zirconia is also probed using microsecond long molecular dynamics. Contrary to assumption frequently made in literature that vacancies move via hop mechanism, we find that a significant proportion of the moves involve concerted motion of vacancies, i.e. several mechanisms contribute to the ionic conductivity. The relevance of these mechanisms is investigated over a range of compositions (6-10 mol\% YSZ) and temperatures (800-1200K). The effect of bridging cation sites, vacancies and Y^{3+} ions in the local environment on the kinetic rate is discussed. References [1] R. Pornprasertsuk, P. Ramanarayanan, C.B. Musgrave, F.B. Prinz, Predicting ionic conductivity of solid oxide fuel cell electrolyte from first principles, J. Appl. Phys. 98 (2005) 0–8. doi:10.1063/1.2135889.

We consider iron phosphate, as a candidate material for the encapsulation of nuclear wastes. We studied amorphous and crystalline structures of Fe2+Fe3+2(P2O7)2 and Fe3+(PO3)3. Amorphous Fe3+4(P2O7)3, and two structures comparable with experimentally produced iron phosphate glass (IPG): 40 mol % Fe2O3 and 60 mol % P2O5, with 4% and 17% Fe2+ ion concentrations, were also investigated. We investigated the radiation resistance of these materials by performing constant volume molecular dynamics (MD) simulations of radiation cascades at 4 keV. The cascades caused a localised thermal spike that displaced up to 600 atoms. We also observed channelling, of typically oxygen atoms, that cause branching sub-cascades. We analysed the local radiation damage by considering the coordination number of each ion, during a cascade. We found that in all cases, many PO4 tetrahedra are initially destroyed, but recover within 1 ps after the cascade. To analyse the longer range damage, we consider the chains of PO4 tetrahedra within each structure. We found that, in the crystal structures, the radiation damage cascade caused the local region to become amorphous. We also calculated the threshold displacement energies for each structure. We found that the threshold displacement energy in the Fe3+(PO3)3 crystal is significantly higher than all other cases. Furthermore, this material also contained far fewer displaced atoms in its damaged structure. Our work suggests that iron phosphate glass (IPG) with a low Fe2+ ion content, is a good candidate for radioactive waste immobilisation.

The production of low-e glass includes a thin silver Ag layer used to reflect infra-red radiation. This thin film is often grown on a ZnO substrate via magnetron sputtering techniques. We examine the growth of a thin silver layer on a zinc oxide substrate, via Molecular Dynamics and Long Time Scale Dynamics (on-the-fly Kinetic Monte Carlo). A specially constructed ReaXFF variable charge potential model was developed to model this process. As silver is an expensive material, optimising thin film growth on a zinc oxide whilst retaining optical performance is financially beneficial to industry and the interface between the Ag and ZnO layers have been shown to be one of the weakest in the multilayer due to a relatively large mismatch and low adhesion energy. Thus, finding improvements in adhesion along with reducing the quantity of silver in the coatings is of great interest. Initial growth phases have been simulated on perfect and defective ZnO surfaces for low deposition energies. Results indicate that surface defects caused by oxygen deficiencies repel Ag adatoms whilst step edges attract. These characteristics are examined throughout initial growth phases with the overall aim of finding the optimum environment to grow a smooth silver thin film.

The inhibition of dislocations motion by irradiation-induced defects, such as dislocation loops, is one of the main mechanisms of irradiation hardening of austenitic stainless steels. In this work, Molecular Dynamics (MD) simulations of interaction between an edge dislocation and Frank loops in FeeNi10eCr20 ternary alloy mimicking austenitic stainless steels are carried out to investigate and model dislocation behavior. An empirical interatomic potential developed recently for a ternary FeNiCr system is used for the MD calculations. The interactions are calculated at different temperatures, loop orientations, loop size and solute atom configurations. The results show that the loop strength and the interaction pro- cesses depend on the solute atom configuration, the geometrical configurations between the dislocation and the loop and temperature. It is also demonstrated that a small Frank loop is not so weak an obstacle in the alloy. The interaction leads microstructural change such as loop shearing, loop unfaulting and loop absorption in the dislocation. In the former two cases, the loop remains after the interaction, however in some cases an absorption of the remaining loop by subsequent interactions with successive dislocations is observed.

Vacancy migration is studied in a silicon crystal with atomic interactions described by the Kumagai potential [1]. The basic functional form of this potential is very similar to the Tersoff potential. The main improvements concern the values of the elastic constants and the melting temperature. However, the potential energy surface is as rough as in the case of the Tersoff potential. In the ground state the vacancy has the “normal” tetrahedral configuration. The migration in the rugged potential energy landscape leads to peculiarities. Extensive Molecular Dynamics (MD) calculations show that the atomic mechanism of the migration process depends on temperature. The vacancy migration energy changes at about 1000 K: At lower and higher temperatures it is about 0.71 and 0.41 eV, respectively. Investigations on the dominating defect structure show that below about 1000 K the tetrahedral vacancy prevails whereas at higher temperature a modified version of the tetrahedral vacancy, the split vacancy and other configurations become dominant. Applying the Nudged Elastic Band (NEB) method to the potential energy surface in the ground state, the transition between neighboring tetrahedral vacancy structures was studied. A number of intermediate metastable states were found, amongst them the modified version of the tetrahedral vacancy and the split vacancy. The maximum barrier for the migration between one ground state configuration to another is about 0.9 eV, whereas a barrier of about 0.3 eV is found for the transition from the split to the modified tetrahedral structure. Comparing with the results of MD simulations one may assume that in the high temperature range the vacancy moves mainly between high-energy configurations such as the split and the modified tetrahedral structure. The reason why the vacancy is not found in the ground state is not completely clear. Obviously, the free energy landscape at elevated temperature differs strongly from the ground-state energy landscape. Vibrational degrees of freedom may lead to the narrowing of the path to the tetrahedral state while the path between the high energy states may become much broader. [1] T. Kumagai et al., Comput. Mater. Sci. 39, 457 (2007)

The performance of a recently proposed method to efficiently calculate scar functions is analyzed in problems of chemical interest. An application to the computation of wave functions associated with barriers relevant for the LiNC$\rightleftarrows$LiCN isomerization reaction is presented as an illustration. These scar functions also constitute excellent elements for basis sets suitable for quantum calculation of vibrational energy levels. To illustrate this efficiency a calculation of the LiNC/LiCN eigenfunctions is also presented.

Silver thin films are used in the glass industry to produce low emissivity (low-E) coatings. In summer the coating can block solar radiation from the outside and in winter keep the heat inside.  Silver-based low-E coatings have high conductivity and good optical properties such as higher transmittance and low absorption. The basic stack of a low-E coating contains a seed layer/silver/barrier layer and the motivation for the work is to investigate the barrier layer, which can involve the growth of Ti on an Ag surface.  Since growth is usually carried out by magnetron sputtering, a good model for the Ti-Ag system is required both for the surface energies of the two metals and also for the way in which the atoms and small molecules attach to the surface. Two interatomic potential mixing rules for the Ti-Ag system developed by Johnson [1] and Ward [2] were investigated based on embedded atom method (EAM) elemental potentials proposed by Wadley [3].  Deposition of single Ti atoms on Ag surfaces and single Ag atoms on Ti surfaces was undertaken using molecular dynamics (MD) simulations but the two mixing rules give contrasting results for the preferred deposition sites.   As a result, first principles calculations were performed via SIESTA [4] for various configurations of the Ti-Ag system to see which model best fitted the ab initio results.  The results showed that the surface energies, especially that of Ti, were not well fitted by either model and there were discrepancies in the ad-atom energies.  As a result, the modified embedded atom method (MEAM) [5] was investigated. In contrast to the other models, surface energies for pure Ti calculated by MEAM turned out to be in good agreement with the experimental data and the ab initio results. The MEAM mixing rules were used to investigate Ag ad-atoms on Ti and Ti ad-atoms on Ag. The results showed good agreement with SIESTA, especially after some parameter optimisation. [1] RA Johnson. Alloy models with the embedded-atom method. Physical Review B, 39(17):12554, 1989. [2] L Ward, A Agrawal, KM Flores, and W Windl. Rapid production of accurate embedded-atom method potentials for metal alloys. arXiv preprint arXiv:1209.0619, 2012. [3] XW Zhou, RA Johnson, and HNG Wadley. Misfit-energy-increasing dislocations in vapor-deposited cofe/nife multilayers. Physical Review B, 69(14):144113, 2004. [4] JM Soler, E Artacho, JD Gale, A Garcia, J Junquera, P Ordejon, and D Sanchez-Portal. The siesta method for ab initio order-n materials simulation. Journal of Physics: Condensed Matter, 14(11):2745, 2002. [5] MI Baskes. Modified embedded-atom potentials for cubic materials and impurities. Physical Review B, 46(5):2727, 1992.