Kostya Trachenko

DL_POLY_3: new dimensions in molecular dynamics simulations via massive parallelism

DL_POLY_3 is a general-purpose massively parallel molecular dynamics simulation package embedding a highly efficient set of methods and algorithms such as: Domain Decomposition (DD), Linked Cells (LC), Daresbury Advanced Fourier Transform (DAFT), Trotter derived Velocity Verlet (VV) integration and RATTLE. Written to support academic research, it has a wide range of applications and can run on a wide range of computers; from single processor workstations to multi-processor computers. The code development has placed particular emphasis on the efficient utilization of multi-processor power by optimised memory workload and distribution, which makes it possible to simulate systems of the order of tens of millions of particles and beyond. In this paper we discuss the new DL_POLY_3 design, and report on the performance, capability and scalability. We also discuss new features implemented to simulate highly non-equilibrium processes of radiation damage and analyse the structural damage during such processes.

Understanding resistance to amorphization by radiation damage

Decades of experimental and theoretical studies have brought some useful insights about what defines resistance to amorphization by radiation damage; however, the problem is still viewed as generally unsolved. I review ideas and concepts that have been put forward to help with understanding this problem. I then discuss how the type of interatomic force is relevant for resistance to amorphization, with covalency of bonding stabilizing the damage and making material amorphizable. On a more detailed level, I suggest that resistance to amorphization of a complex non-metallic material is defined by the competition between the short-range covalent and long-range ionic forces. I follow this with a review of experimental data on 116 materials, to illustrate that the type of interatomic force can generally explain the resistance to amorphization. I conclude by discussing how the proposed picture is related to models proposed previously, and by suggesting some possible future research.

The phonon theory of liquid thermodynamics

Heat capacity of matter is considered to be its most important property because it holds information about systems degrees of freedom as well as the regime in which the system operates, classical or quantum. Heat capacity is well understood in gases and solids but not in the third main state of matter, liquids, and is not discussed in physics textbooks as a result. The perceived difficulty is that interactions in a liquid are both strong and system-specific, implying that the energy strongly depends on the liquid type and that, therefore, liquid energy can not be calculated in general form. Here, we develop a phonon theory of liquids where this problem is avoided. The theory covers both classical and quantum regimes. We demonstrate good agreement of calculated and experimental heat capacity of 21 liquids, including noble, metallic, molecular and hydrogen-bonded network liquids in a wide range of temperature and pressure.

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.

Liquid-gas transition in the supercritical region: fundamental changes in the particle dynamics.

Recently, we have proposed a new dynamic line on the phase diagram in the supercritical region, the Frenkel line. Crossing the line corresponds to the radical changes of system properties. Here, we focus on the dynamics of model Lennard-Jones and soft-sphere fluids. We show that the location of the line can be rigorously and quantitatively established on the basis of the velocity autocorrelation function (VAF) and mean-square displacements. VAF is oscillatory below the line at low temperature, and is monotonically decreasing above the line at high temperature. Using this criterion, we show that the crossover of particle dynamics and key liquid properties occur on the same line. We also show that positive sound dispersion disappears in the vicinity of the line in both systems. We further demonstrate that the dynamic line bears no relationship to the existence of the critical point. Finally, we find that the region of existence of liquidlike dynamics narrows with the increase of the exponent of the repulsive part of interatomic potential.

Heat capacity of liquids: An approach from the solid phase

We calculate the energy and heat capacity of a liquid on the basis of its elastic properties and vibrational states. The experimental decrease of liquid heat capacity with temperature is attributed to the increasing loss of two transverse modes with frequency 1/, where is liquid relaxation time. In a simple model, liquid heat capacity is related to viscosity and is compared with the experimental data of mercury. We also calculate the vibrational energy of a quantum liquid, and show that transverse phonons cannot be excited in the lowtemperature limit. Finally, we discuss the implications of the proposed approach to liquids for the problem of glass transition.

Thermodynamic behaviour of supercritical matter

Since their discovery in 1822, supercritical fluids have been of enduring interest and have started to be deployed in many important applications. Theoretical understanding of the supercritical state is lacking and is seen to limit further industrial deployment. Here we study thermodynamic properties of the supercritical state and discover that specific heat shows a crossover between two different regimes, an unexpected result in view of currently perceived homogeneity of supercritical state in terms of physical properties. We subsequently formulate a theory of system thermodynamics above the crossover, and find good agreement between calculated and experimental specific heat with no free-fitting parameters. In this theory, energy and heat capacity are governed by the minimal length of the longitudinal mode in the system only, and do not explicitly depend on system-specific structure and interactions. We derive a power law and analyse supercritical scaling exponents in the system above the Frenkel line.

Real-space rigid-unit-mode analysis of dynamic disorder in quartz, cristobalite and amorphous silica

We use a recently developed tool based on geometric algebra to analyse the phase transition in quartz, the nature of the disordered high-temperature phase of cristobalite and the dynamics of silica glass. The approach is to analyse configurations generated by the reverse Monte Carlo or molecular dynamics simulations in terms of rigid-unit-mode (RUM) motions, but concentrating on quantifying the real-space distortions rather than performing a reciprocal-space analysis in terms of RUM phonons. One of the important results is a measure of the extent to which the amplitudes of motion are directly attributable to RUMs, and how the RUM fraction changes as a result of a phase transition.

Refinement of the Si?O?Si bond angle distribution in vitreous silica

A model of silica glass consisting of a fully connected corner-sharing network of SiO4 tetrahedra is refined using neutron diffraction data and reverse Monte Carlo modelling. This model is then used to investigate optimal inter-tetrahedral Si–O–Si bond angle distributions. The distribution which is most consistent with the data is found to be centred around θSi−O−Si = 151.0° with a standard deviation of between 9° and 12°. Other recent determinations of the Si–O–Si bond angle distribution are in good agreement with this result.

Collective modes and thermodynamics of the liquid state.

Strongly interacting, dynamically disordered and with no small parameter, liquids took a theoretical status between gases and solids with the historical tradition of hydrodynamic description as the starting point. We review different approaches to liquids as well as recent experimental and theoretical work, and propose that liquids do not need classifying in terms of their proximity to gases and solids or any categorizing for that matter. Instead, they are a unique system in their own class with a notably mixed dynamical state in contrast to pure dynamical states of solids and gases. We start with explaining how the first-principles approach to liquids is an intractable, exponentially complex problem of coupled non-linear oscillators with bifurcations. This is followed by a reduction of the problem based on liquid relaxation time τ representing non-perturbative treatment of strong interactions. On the basis of τ, solid-like high-frequency modes are predicted and we review related recent experiments. We demonstrate how the propagation of these modes can be derived by generalizing either hydrodynamic or elasticity equations. We comment on the historical trend to approach liquids using hydrodynamics and compare it to an alternative solid-like approach. We subsequently discuss how collective modes evolve with temperature and how this evolution affects liquid energy and heat capacity as well as other properties such as fast sound. Here, our emphasis is on understanding experimental data in real, rather than model, liquids. Highlighting the dominant role of solid-like high-frequency modes for liquid energy and heat capacity, we review a wide range of liquids: subcritical low-viscous liquids, supercritical state with two different dynamical and thermodynamic regimes separated by the Frenkel line, highly-viscous liquids in the glass transformation range and liquid-glass transition. We subsequently discuss the fairly recent area of liquid-liquid phase transitions, the area where the solid-like properties of liquids have become further apparent. We then discuss gas-like and solid-like approaches to quantum liquids and theoretical issues that are similar to the classical case. Finally, we summarize the emergent view of liquids as a unique system with a mixed dynamical state, and list several areas where interesting insights may appear and continue the extraordinary liquid story.