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). Cohesive energies calculated by vasp are with respect to this configuration. The reference calculation, however, did not allow for spin-polarisation or broken symmetry solution. 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 ISMEAR = 0 ! Gaussian smearing, otherwise negative occupancies SIGMA = 0.1 ! intermid. smearing width AMIX = 0.2 ! mixing set manually BMIX = 0.0001 NELM = 20 ! 20 electronic steps ICHARG = 1Execute VASP twice, consecutively with this input file to get converged energies.