Michael Seaton

DL_MESO: highly scalable mesoscale simulations

DL_MESO is a parallel mesoscale simulation package capable of dissipative particle dynamics and the lattice Boltzmann equation method. It has been developed at Daresbury Laboratory for the United Kingdom Collaborative Computational Project known as CCP5. Capable of addressing industrially relevant tasks, but written to support academic research, it has a wide range of applications and scales to thousands of processors on high-performance computing platforms yet runs efficiently on smaller commodity clusters and single processor personal computers. This article serves as a guide to a variety of users, describing the functionality, performance and structure of this simulation package. Representative examples highlighting the capabilities of DL_MESO are given for each of the two methodologies available. Future directions for the package are discussed towards the end of the article.

The nature of high-energy radiation damage in iron

Understanding and predicting a materials performance in response to high-energy radiation damage, as well as designing future materials to be used in intense radiation environments, requires knowledge of the structure, morphology and amount of radiation-induced structural changes. We report the results of molecular dynamics simulations of high-energy radiation damage in iron in the range 0.2-0.5 MeV. We analyze and quantify the nature of collision cascades both at the global and the local scale. We observe three distinct types of damage production and relaxation, including reversible deformation around the cascade due to elastic expansion, irreversible structural damage due to ballistic displacements and smaller reversible deformation due to the shock wave. We find that the structure of high-energy collision cascades becomes increasingly continuous as opposed to showing sub-cascade branching as reported previously. At the local length scale, we find large defect clusters and novel small vacancy and interstitial clusters. These features form the basis for physical models aimed at understanding the effects of high-energy radiation damage in structural materials.

Electronic effects in high-energy radiation damage in iron

Electronic effects have been shown to be important in high-energy radiation damage processes where a high electronic temperature is expected, yet their effects are not currently understood. Here, we perform molecular dynamics simulations of high-energy collision cascades in α-iron using a coupled two-temperature molecular dynamics (2T-MD) model that incorporates both the effects of electronic stopping and electron-phonon interaction. We subsequently compare it with the model employing electronic stopping only, and find several interesting novel insights. The 2T-MD results in both decreased damage production in the thermal spike and faster relaxation of the damage at short times. Notably, the 2T-MD model gives a similar amount of final damage at longer times, which we interpret to be the result of two competing effects: a smaller amount of short-time damage and a shorter time available for damage recovery.

High-energy radiation damage in zirconia: Modeling results

Zirconia is viewed as a material of exceptional resistance to amorphization by radiation damage, and consequently proposed as a candidate to immobilize nuclear waste and serve as an inert nuclear fuel matrix. Here, we perform molecular dynamics simulations of radiation damage in zirconia in the range of 0.1–0.5 MeV energies with account of electronic energy losses. We find that the lack of amorphizability co-exists with a large number of point defects and their clusters. These, importantly, are largely isolated from each other and therefore represent a dilute damage that does not result in the loss of long-range structural coherence and amorphization. We document the nature of these defects in detail, including their sizes, distribution, and morphology, and discuss practical implications of using zirconia in intense radiation environments.

Electronic effects in high-energy radiation damage in tungsten.

Although the effects of the electronic excitations during high-energy radiation damage processes are not currently understood, it is shown that their role in the interaction of radiation with matter is important. We perform molecular dynamics simulations of high-energy collision cascades in bcc-tungsten using the coupled two-temperature molecular dynamics (2T-MD) model that incorporates both the effects of electronic stopping and electron-phonon interaction. We compare the combination of these effects on the induced damage with only the effect of electronic stopping, and conclude in several novel insights. In the 2T-MD model, the electron-phonon coupling results in less damage production in the molten region and in faster relaxation of the damage at short times. These two effects lead to a significantly smaller amount of the final damage at longer times.

DL_MESO_DPD: development and use of mesoscale modelling software

dl_meso is a highly-scalable general purpose software package for mesoscale modelling. Created and developed at Daresbury Laboratory for the UK Collaborative Computational Project CCP5, it was inte...

Domain decomposition of the two-temperature model in dl_poly_4

ABSTRACTIncluding the effects of excited electrons in classical simulations, at the level of the two-temperature model, involves the coupling of a grid-based finite-difference solver for a heat diffusion equation and classical molecular dynamics simulations with an inhomogeneous thermostat. Simulation of large systems requires domain decomposition of both particle-based and grid-based techniques. Starting with the CCP5 flagship code dl_poly_4 as the domain-decomposed molecular dynamics code, we devised a method to divide up temperature grids among processor cores in a similar fashion, including the appropriate communications between cores to deal with boundaries between grid divisions. This article gives the outline of how the domain decomposition of the temperature grids was achieved, as well as some example applications of the two-temperature model implementation in dl_poly_4.

Micelle Formation in Alkyl Sulfate Surfactants Using Dissipative Particle Dynamics

We use dissipative particle dynamics (DPD) to study micelle formation in alkyl sulfate surfactants, with alkyl chain lengths ranging from 6 to 12 carbon atoms. We extend our recent DPD force field [ J. Chem. Phys. 2017 , 147 , 094503 ] to include a charged sulfate chemical group and aqueous sodium ions. With this model, we achieve good agreement with the experimentally reported critical micelle concentrations (CMCs) and can match the trend in mean aggregation numbers versus alkyl chain length. We determine the CMC by fitting a charged pseudophase model to the dependence of the free surfactant on the total surfactant concentration above the CMC and compare it with a direct operational definition of the CMC as the point at which half of the surfactant is classed as micellar and half as monomers and submicellar aggregates. We find that the latter provides the best agreement with experimental results. Finally, with the same model, we are able to observe the sphere-to-rod morphological transition for sodium dodecyl sulfate (SDS) micelles and determine that it corresponds to SDS concentrations in the region of 300-500 mM.

Chapter 23 – Vectorization Advice

This chapter introduces a tool that radically improves the ease at which you can analyze the nature of the vectorization in the hot loops of a program. The discoveries this tool helps make on how compiled code is vectorized, or what is stopping code from being vectorized, checking for loop dependencies, and observing the memory access patterns, are of value regardless of what system you are targeting. The examples provided in the chapter, clearly show what can be achieved using the Vector Advisor.

Efficient Domain Decomposition of Dissipative Particle Dynamics via Choice of Pseudorandom Number Generator

Domain decomposition of dissipative particle dynamics is complicated by the use of random pairwise forces as a component of a momentum-conserving thermostat. The conventional use of a pseudorandom number generator for each processor core leads to the need for an additional communication step to correctly assign random forces to particles in boundary halos. To circumvent this communication, the use of a three-seed pseudorandom number generator is proposed to allow multiple processor cores to evaluate the same forces. This kind of pseudorandom number generator will be applied to the general-purpose mesoscale modelling package DL_MESO to improve its parallel scalability for large processor core counts.