Extracting convergent surface formation energies from slab calculations

It is known that surface formation energies obtained as differences between slab energies and an independently determined bulk energy will diverge linearly with the slab thickness. A recent paper by Fiorentini and Methfessel presented `a solution to this problem that eliminates the divergence and leads to rapidly convergent and accurate surface energies'. Although their work is correct, the solution they propose is not new. In this comment, we provide some of the historical background neglected by Fiorentini and Methfessel.