On the validity of empirical potentials for simulating radiation damage in graphite: a benchmark

C D Latham,M J Rayson,T P Trevethan,A J Mckenna,P R Briddon,M I Heggie

doi:10.1088/0953-8984/27/31/316301

Abstract

In this work, the ability of methods based on empirical potentials to simulate the effects of radiation damage in graphite is examined by comparing results for point defects, found using ab initio calculations based on density functional theory (DFT), with those given by two state of the art potentials: the Environment-Dependent Interatomic Potential (EDIP) and the Adaptive Intermolecular Reactive Empirical Bond Order potential (AIREBO). Formation energies for the interstitial, the vacancy and the Stone–Wales (5775) defect are all reasonably close to DFT values. Both EDIP and AIREBO can thus be suitable for the prompt defects in a cascade, for example. Both potentials suffer from arefacts. One is the pinch defect, where two α-atoms adopt a fourfold-coordinated sp3 configuration, that forms a cross-link between neighbouring graphene sheets. Another, for AIREBO only, is that its ground state vacancy structure is close to the transition state found by DFT for migration. The EDIP fails to reproduce the ground state self-interstitial structure given by DFT, but has nearly the same formation energy. Also, for both potentials, the energy barriers that control diffusion and the evolution of a damage cascade, are not well reproduced. In particular the EDIP gives a barrier to removal of the Stone–Wales defect as 0.9 eV against DFT's 4.5 eV. The suite of defect structures used is provided as supplementary information as a benchmark set for future potentials.

Highlights

It is essential to use models that have identical size and geometry when making comparisons, which is the case in the present work. In terms of both formation energy and structure, it can be seen from the results presented in table 5 that both potentials apparently perform well with respect to density functional theory (DFT), with Environment-Dependent Interatomic Potential (EDIP) being the winner by a small margin
Owing to the approximate nature of empirical potentials, and the compromises involved in their construction, it is necessary to identify their limitations in order to justify their use
The purpose of the present work is to provide a benchmark for carbon potentials, that is applicable to simulations of radiation damage in graphite at the atomic scale

Summary

Introduction

Is sufficient to displace every carbon atom in a specimen tens of times. This resilience, together with several other properties of graphite, make it attractive for applications in the nuclear industry. The world’s first artificial nuclear fission reactor used blocks of graphite for the moderator and reflector material. It is still used in present-day reactors for this purpose, and appears often in designs proposed for the future. Since the modification of properties that does occur upon exposure to damaging radiation may affect the performance of operating nuclear reactors, radiation damage in graphite has remained a subject of interest since the 1940s

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