. Also, atomistic computer simulations of hydrogen decohesion effects have been undertaken based on models of Fe-Fe and Fe-H pair potentials, as described in the review by Markworth and Halbrook 8. This approach is critically dependent on realistic models of atom pair potentials. Equally important, Gerberich at al. 9 conducted computer simulations of the elastic stress field near a crack tip in iron and found that local hydrostatic tensile stresses could be extremely high, approaching ~20 GPa. They concluded that these stresses were sufficient to produce very high local concentrations of hydrogen solute, even when the initial lattice hydrogen concentration was very dilute.It is now evident that the hydrogen embrittlement of iron in many situations is related to the effect of local high concentrations of hydrogen on the cohesive energy of the iron lattice. However, this does not negate considering hydrogen embrittlement from the surface energy viewpoint, because bond energy and surface energy are inter-related. This relationship is evident from the studies of Mackenzie et al. 10 who related the anisotropy of surface energy with the number of broken bonds per unit area, and the work of Tyson 11, who showed that the problem of calculating surface energy is closely related to the problem of calculating the cohesive energy.””The idea is that dissolved hydrogen lowers the cohesive energy of the iron lattice because the interatomic distance increases due to the filling of the d bands of the transition metal by the electron of the hydrogen atom as shown in Fig. 1.10 35. Recently, first principles calculations by 36 showed that the hydrogen reduces the cohesive energy in cleavage fracture on the usual.(100) planes of single crystal ?-iron. The decohesion theory for the hydrogen-induced crack propagation 34 postulates that the highly elastically stressed region at the crack front lowers sufficiently the chemical potential of dissolved hydrogen, which then attains a concentration that is several orders of magnitude larger than normal. This in turn lowers the maximum cohesive energy between the atoms. Cracks propagate when the local maximum tensile stress normal to the plane of the crack, which is controlled by the externally applied load and the crack-front geometry, is equal to the maximum cohesive energy.