A Modified Two-Temperature Molecular Dynamics for Simulating Radiation Damage Cascades

Two-temperature molecular dynamics (2T-MD) is a modelling paradigm in which a traditional molecular dynamics simulation, based on effective interatomic potentials, is married to a continuum representation of an associated electron gas. An overarching aim of the method is to provide an account of electronic excitation effects in what is otherwise an electron-less theory of atomic movements&lt;!-- I suggest : “what is otherwise an electron-less theory of atomic movements”. Speaking of ‘ground state atoms’ in a cascade context sounds weird to me. --&gt;. The model has previously been applied to model swift heavy ion irradiation, ultra-fast laser heating of solids, and electronic effects in radiation damage cascades.<br/>Heat exchange between ions and electrons (and vice versa) is permitted by inclusion of electron-phonon coupling (often taken as input from prior density functional theory calculations), with high energy ions subject to an additional electronic stopping force. Recent research has shown that the effect of electronic excitations, modelled via 2T-MD, on the evolution of radiation damage cascades in solids is significant, and leads to a reduction of quantitative damage estimates in some materials by around a factor of two.<br/>Here, we present our modifications to the 2T-MD model that are aimed at providing a more realistic description of the interplay between ionic and electronic degrees of freedom in the short time after the instantiation of a radiation damage event. During this time, the dynamics are out-of-equilibrium, and the standard conditions for electron-phonon coupling do not apply. By removing the disproportionate, improper contribution of primary knock-on atoms from the ionic temperature estimator, we are able to remedy the excessive electron-phonon coupling problem which previously required artificial removal from cascade calculations. The definition of an ionic temperature, for instance, is no longer strictly possible, and care must be taken to limit the degree of electron-phonon coupling in this short-lived out-of-equilibrium phase of the dynamics. We discuss the nature and the impacts of our modifications, with particular emphasis on Tungsten, and ramifications for its putative use as a first-wall and divertor material in upcoming fusion reactor projects.