Relation between Plane Wave Cutoff and Pseudopotential Files

It is important to note that the cutoff value depends on the individual pseudpotential and its parameterization and that in more complex system the ``hardest'' pseudopotential usually discriminates the required cutoff. Two additional ``rules of the thumb'' are that a) the cutoff requirements usually increases with the valence charge, and b) that for pseudopotentials with the same valence charge, the heavier elements are usually ``softer'' (i.e. require not as large a cutoff). Also different pseudopotential-``families'' (SGS, BHS, MT, VDB, SG, GH) have different requirements due to the functional form used. Ultra-soft (VDB) pseudopotentials are a special case, since their wavefunction cutoff requirements are generally low (in the 25-40ryd range), but the correct representation of augmentation charges requires an increased DENSITY CUTOFF (150-500ry), frequently also specified as ratio relative to the wavefunction cutoff via the DUAL keyword.

Finally, one has to keep in mind that different parameters of a system under consideration (e.g. total energy, forces, stress tensor, or the energy difference between important configurations) can have different convergence behavior.