In this definition the number of atoms at the surface is proportional to the area. Gibbs was the first to define another surface quantity, different from the surface free energy γ γ , that is associated with the reversible work per unit area needed to elastically stretch a pre-existing surface. In a continuum approach one can define a surface stress tensor f i j that relates the work associated with the variation in γ γ A , the total excess free energy of the surface due to a strain tensor e i j