Relation between thermal expansion and interstitial formation energy in pure Fe and Cr

By fitting a potential of modified Finnis-Sinclair type to the thermal expansion of ferromagnetic Fe and paramagnetic Cr, stability of the 〈1 1 0〉 self-interstitial atom is obtained. The resulting potentials are relatively hard, yielding high SIA formation energies. Less hard potentials give lower interstitial formation energy, but predict too small thermal expansion. We also show that the formation energy of the 〈1 1 1〉 SIA depends on distances in-between the 2nd and 3rd neighbour. By raising the value of the pair potential in this region, the energy difference with respect to the 〈110〉 configuration calculated with VASP in the PAW approximation can be reproduced.