In the last decade, especially since the development of the density functional theory (DFT), first-principle methods have dramatically improved our knowl­edge of point defect and diffusion properties in metals.69 They provide a reliable way to compute the formation and binding energies of defects, their equilibrium configuration and migration barriers, the influence of the local atomic configuration in alloys, etc. Migration energies are usually com­puted by the drag method or by the nudged elastic band methods. The DFT studies on self-interstitial properties — for which few experimental data are available — are of particular interest and have recently contributed to the resolution of the debate on self-interstitial migration mechanism in a-iron.70,71 However, the knowledge is still incomplete; calcula­tions of point defect properties in alloys remain scarce (again, especially for self-interstitials), and, in general, very little is known about entropic contributions. Above all, DFT methods are still too time consuming to allow either the ‘on-the-fly’ calculations of the migration barriers, or their prior calculations, and tab­ulation for all the possible local configurations (whose
number increases very rapidly with the range of inter­actions and the number of chemical elements). More approximate methods are still required, based on para­meters which can be fitted to experimental data and/or ab initio calculations.

1.18.3.4.1.1 Interatomic potentials

Empirical or semiempirical interatomic potentials, currently developed for molecular dynamics simula­tions, can be used for the modeling of RIS, but two problems must be overcome:

• To get a reliable description of an alloy, the inter­atomic potential should be fitted to the properties that control the flux coupling of point defects and chemical elements. A complete fitting procedure would be very tedious and, to our knowledge, has never been achieved for a given system.

• The direct calculation of migration barriers with an interatomic potential, even though much sim­pler than DFT calculations, is still quite time con­suming. Full calculations of vacancy migration barriers have indeed been implemented in Monte Carlo simulations,72 using massively parallel cal­culation methods, but they are still limited to rela­tively small systems and short times, for example, for the study of diffusion properties rather than microstructure evolution. It is possible to simplify the calculation of the jump frequency, for example, by not doing the full calculation of the attempt frequencies (their impact on the jump frequency must be less critical than that of the migration barriers involved in the exponential term) or by the relaxation of the saddle-point position.73 Malerba et al. have recently proposed another method where the point defect migration barriers of an interatomic potential are exactly computed for a small subset of local configurations, the others being extrapolated using artificial intelli­gence techniques. This has been successfully used for the diffusion of vacancies in iron-copper alloys.74,75 Such techniques have not yet been used to model RIS phenomena, but this could change in the future.