Radiation damage in the bulk and at the surface

The UK environmental e-science initiative supports the development and modification of simulation tools used to study radiation damage effects. We discuss the development and modification to the DL_POLY molecular dynamics (MD) code. Using the newly developed tools, we study the effects of radiation damage related to the safe encapsulation of highly radioactive materials, including nuclear waste. We address the possible differences between the radiation damage in the bulk and at the surface of a material, and perform MD simulations of energetic events in zircon structure. We find that in the case of readily amorphizable material, the formation of a stable alternative covalent network reduces the possible effect of the surface on the damaged structure.     

A short description of DL_POLY

DL_POLY is a general purpose molecular dynamics simulation package with in-built parallel algorithms. It may be run on a wide selection of distributed memory parallel computers, from national supercomputers with thousands of processors, to single processor workstations and can simulate small systems with order 100 atoms, to systems with millions of atoms. This introduction provides an outline of the features of the package and the underlying methodology.     

Simulations of melting of polyatomic solids and nanoparticles

Molecular dynamics (MD) methods for calculating the melting point of complex molecular and ionic solids and nanoparticles are described. Various approaches for simulating melting and computing the thermodynamic melting point are discussed along with some force fields that have been used in simulations of the melting of molecular and ionic solids. The different structural, energetic and dynamical quantities used to characterize the melting transition are described. The article ends with a discussion of selected examples of melting point calculations of bulk solids and nanoparticles. Pointers on how each method can be implemented in DL_POLY are given.     

Computational polymer dynamics via DL_POLY

A review is presented of recent work in the group utilising the DL_POLY molecular dynamics package. Simulations have been performed across several areas of polymer physics including the dynamics of polyethylene as a function of density and chain length, the rheology of trifunctional dendrimers, and penetrant diffusion with a macromolecular matrix. The methodology and conclusions of these investigations are reviewed.     

Application of molecular dynamics DL_POLY codes to interfaces of inorganic materials

Three recent applications of the DL_POLY molecular dynamics code are described, which demonstrate the flexibility and viability of the code for extending our understanding of the structure, stability and reactivity of ceramics and minerals at the atomic level. The first is an investigation into differences in oxygen atom mobility in bulk and at the most stable {111} surface of ceria. The results show enhanced surface transport but that it is via subsurface oxygen. Secondly, we investigate how polychloro-dibenzo-pdioxins (PCDDs) molecules might adsorb on clay surfaces. The resulting adsorption energies show a clear relationship with chlorine content of the molecule. Finally, we apply DL_POLY to comparing the aggregation of magnesium oxide and calcium carbonate nanoparticles. We find that very small calcium carbonate nanoparticles are amorphous and their aggregation shows no preferred orientation in contrast to magnesium oxide, which remain highly crystalline and combine in a highly structural specific way.     

Applications of DL_POLY to modelling of mesoscopic particulate systems

The molecular dynamics package DL_POLY has at its heart a number of versatile and efficient dynamics algorithms that can readily be adapted to extend the application of this code well beyond the time and length scales typically associated with atomistic simulations. In order to achieve this, it is necessary to substitute the appropriate interparticle potentials and forces in place of the default functional forms in DL_POLY, which are mainly suitable for molecular systems. To facilitate this, it may be required to incorporate additional factors, into the simulation, such as velocity-dependent dissipation effects (friction), rotational degrees of freedom and non-spherosymmetric forces. In this paper, we will discuss some of the practical details of implementing these changes to DL_POLY (version 2) together with applications of discrete particle dynamics methods, such as dissipative particle dynamics (DPD) and granular dynamics (GD) (also known as the discrete or distinct element method, DEM) to particle packing in composite systems and pharmaceutical powders. We also consider how well the approach of simulating particles of arbitrary shape using rigid assemblies of fused soft spheres (each individually interacting via pairwise continuous potentials) compares to true hard-body simulations of polygonal particles.