In steels, the presence of carbon, even though its concentration is very low, considerably affects de­fect properties because of the strong carbon-defect

interaction. DFT calculations reproduce the well — known fact that carbon is located in octahedral sites, and they also confirm the strong attraction between interstitial carbon and a monovacancy, with a binding energy of about 0.5 eV.72-74 This strong attraction is the origin of the confusing discrepancy between the vacancy migration energy in ultrapure iron, ^0.6 eV, and the effective vacancy migration energy in iron with carbon or in steels, that is, ~1.1 eV, which cor­responds to first order to the sum of the vacancy migration energy and the carbon-vacancy binding energy.74 More interestingly, DFT calculations pre­dict that the complex formed by a vacancy and two carbon atoms, VC2, is extremely stable, due to the formation of a strong covalent bond between the carbon atoms. The VC-C binding energy is indeed close to 1 eV,72-74 and VC2 complexes are expected to play a very important role.

The interaction between carbon and self­interstitials is also attractive but weaker. In agreement with experiments,75 DFT calculations confirmed a binding energy of ^0.2 eV76 and predict, at vari­ance with initial empirical potential results, that the nearest-neighbor configurations are repulsive and that the most attractive configuration is that shown in Figure 7. This shortcoming of empirical potentials was overcome recently with an improved potential derived taking into account information from the electronic structure.77

The strong interaction of carbon with vacancies also affects the energetics of helium-vacancy clusters, and it is important to take this into account to repro­duce, for example, thermal helium desorption experi­ments performed in iron.78

Similar calculations have been performed with

72

nitrogen.