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How Good is Damped Molecular Dynamics as a Method for Simulating Radiation Damage in Metals?

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Classical molecular dynamics (MD) is often used to study radiation damage cascades because it provides information on time and length scales inaccessible to experiment. In a radiation damage process, energy transfer from ions to electrons may be important, but there is continued uncertainty over how to incorporate such effects in MD. We use quantum mechanical simulations to evaluate different approaches. The irreversible transfer of energy from fast ions to electrons does not take place in conventional Born-Oppenheimer quantum MD simulations, but can be studied using Ehrenfest-based quantum MD. Our Ehrenfest simulations suggest that a damping force proportional to velocity is sufficient to model energy transfer from ions to electrons in most low energy cascades. We also find, however, that a larger rate of energy transfer is seen when the ionic kinetic energy is confined to a focused sequence of collisions. A viscous damping coefficient dependent on the local atomic environment is shown to be an excellent model for electronic energy losses in low energy cascades in metals.

Bibliography

“Electronic Excitations and their Effect on the Interionic Forces in Simulations of Radiation Damage in Metals”, C.P. Race, D.R. Mason and A.P. Sutton, J. Phys.: Condens. Matter 21, 115702 (2009)

“How Good is Damped Molecular Dynamics as a Method to Simulate Radiation Damage in Metals?” J. le Page, D.R. Mason, C.P. Race, and W.M.C. Foulkes New J. Phys. {\bf 11}, 013004-1 (2009)

“The Ehrenfest Approximation for Electrons Coupled to a Phonon System” J. le Page, D.R. Mason and W.M.C. Foulkes J. Phys.: Condens.~Matter 20, 125212-1 (2008)

“Electronic Damping of Atomic Dynamics in Irradiation Damage of Metals” D.R. Mason, J. le Page, C.P. Race, W.M.C. Foulkes, M.W. Finnis, and A.P. Sutton J. Phys.: Condens. Matter 19, 436209-1 (2007)

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