Specifies the global break condition for the electronic SC-loop. The relaxation of the electronic degrees of freedom will be stopped if the total (free) energy change and the band structure energy change ('change of eigenvalues') between two steps are both smaller than EDIFF. For EDIFF=0 always NELM electronic SC-steps will be performed.
Mind: In most cases the convergence speed is exponential. So if you want the total energy to 4 significant figures set EDIFF to . There is no real no reason to use a much smaller number.