  Recent density functional theory (DFT) calculations by Foerst et al. have
predicted that vacancies in both low and high carbon steels have a carbon dimer
bound to them. This is likely to change the thinking of metallurgists in the
kinetics of the development of microstructures. While the notion of a C2
molecule bound to a vacancy in Fe will potentially assume a central importance
in the atomistic modeling of steels, neither a recent tight binding (TB) model
nor existing classical interatomic potentials can account for it. Here we
present a new TB model for C in Fe, based on our earlier work for H in Fe,
which correctly predicts the structure and energetics of the carbon dimer at a
vacancy in Fe. Moreover the model is capable of dealing with both concentrated
and dilute limits of carbon in both bcc-Fe and fcc-Fe as comparisons with DFT
show. We use both DFT and TB to make a detailed analysis of the dimer and to
come to an understanding as to what governs the choice of its curious
orientation within the vacancy.
