8.5 PAW control tags

In principle the PAW method can be used in the same way as US-PP method. Only special PAW POTCAR files are required. In principle also no additional user interference is required. However there are a few flags that control the behavior of the PAW implementation. The first one is LMAXPAW:

 LMAXPAW = n

This flag controls the maximum L quantum number for the evaluation of the on-site terms on the radial support grids in the PAW method. The default for LMAXPAW is , where is the maximum angular quantum number of the partial waves. Useful settings for LMAXPAW are for instance:

 LMAXPAW = 0

In that case only spherical terms are evaluated on the radial grid. This does not mean that a-spherical terms are totally neglected, because the compensation charges are always expanded up to on the plane wave grid. LMAXPAW=-1 has a special meaning. In that case no correction terms are evaluated on the radial support grid, which effectively means that the behavior of US-PP's is recovered with PAW input datasets. Usually this allows very efficient and fast calculations, and this switch might be of interest for relaxations and molecular dynamics. Energies should be evaluated with the default setting for LMAXPAW.

The second switch, that is useful in the context of the PAW method is ADDGRID. The default for ADDGRID is .FALSE. If

 ADDGRID = .TRUE.

is written in the INCAR file, an additional (third) support grid is used for the evaluation of the augmentation charges. This third grid contains 8 times more points than the fine grid NGXF, NGYF, NGZF. Whenever terms involving augmentation charges are evaluated this third grid is used. For instance: The augmentation charge is evaluated first in real space on this fine grid, ff-transformed to reciprocal space and then added to the total charge density on the grid NGXF, NGYF, NGZF. The additional grid helps to reduce the noise in the forces significantly. In many cases it even allows to perform calculations in which NGXF=NGX etc. This can be achieved by setting

 ENAUG = 1  ; ADDGRID = .TRUE.

in the INCAR file. The flag can also be used for US-PP in particular to reduce the noise in the forces.