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Determining the groundstate energ of atoms

The POTCAR file contains information on the energy of the atom in the reference configuration (i.e. the configuration for which the PP was generated). Total energies calculated by vasp are with respect to this configuration. The reference calculation, however, did not allow for spin-polarisation or broken symmetry solutions, which usually lower the energy for gradient corrected of hybrid functionals. To include these effects properly, it is required to calculate the lowest energy magnetic groundstate using VASP.

Unfortunately convergence to the symmetry broken spin polarized groundstate can be relatively slow in VASP. The following INCAR file worked reasonably well for most elements:

 ISYM = 0      # no symmetry
 ISPIN = 2     # allow for spin polarisation
 VOSKOWN = 1   # this is important, in particular for GGA
               # but not required for PBE potentials
 ISMEAR = 0    # Gaussian smearing, otherwise negative occupancies might come up
 SIGMA = 0.002 # tiny smearing width to safely break symmetry
 AMIX = 0.2    # mixing set manually
 BMIX = 0.0001  
 NELM = 100    # often many steps are required
 ICHARG = 1
Execute VASP twice to three times, consecutively with this input file until energies are converged. Furthermore, we recommend to use large slightly non-cubic cells, i.e. 12 Å$ \times$13 Å$ \times$14 Å. In some cases, we also found it advantageously to use direct energy minimization instead of charge-density mixing
ALGO = D ; LSUBROT = .FALSE. ; NELM = 500 ; TIME = 0.2
or
ALGO = A ; LSUBROT = .FALSE. ; NELM = 500 ; TIME = 0.2
Always check for convergence, and whether all occupancies are 0 or 1.


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N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at