Atoms

About which files do you have to worry:

INCAR
POSCAR
POTCAR
KPOINTS

Before using one of the supplied PAW potentials intensively, it is not only necessary to test the potential for various bulk phases, but the potential also needs to reproduce the eigenvalues and the total energy of the free atom for which it was created. If the energy cutoff and the cell size are sufficiently large, the agreement between the atomic reference calculation (EATOM in the POTCAR file) and a calculation using VASP is usually better than 1 meV (although errors can be 10 meV for some transition metals). In most cases, calculations for a spherical atom are relatively fast and unproblematic. For the calculation the $\Gamma$ point should be used i.e. the KPOINTS file should be similar to

Monkhorst Pack
0
Monkhorst Pack
 1  1  1
 0  0  0

Table 4: Typical convenient settings for the cell size for the calculation of atoms and dimers are (roughly 4-5 times the dimer length):

	cell size
Lithium	13 Å
Aluminium	12 Å
Potassium	14 Å
Copper, Rhodium, Palladium ...	10 Å
Nitrogen	7 Å
C	8 Å

A simple cubic cell is usually recommended; the size of the cell depends on the element in question. Some values for reliable results are compiled in Tab. 4. These cells are also large enough to perform calculations on dimers, explained in the next section. The POSCAR file is similar to:

atom
1
     10.00000    .00000    .00000
       .00000  10.00000    .00000
       .00000    .00000  10.00000
   1
cart
 0    0    0

The INCAR file can be very simple

SYSTEM = Pd: atom

   ENCUT  = 200.00 eV  # energy cut-off for the calculation
   PREC   = Normal     # Normal precision
   LREAL  = .FALSE     ! real space projection .FALSE. or Auto
   ISMEAR =    0; SIGMA=0.1    use smearing method

The only difference to the bulk calculation is that Gaussian smearing should be used. Mind: Extract the correct value for the energy. For atoms and molecules, the value $F=E+ \sigma S$ contains a meaningless entropy term related to orbital degeneracy, and one should rather use the ``energy without entropy'' in the OUTCAR file (when SIGMA is decreased the energy converges to that value).

In some rare cases, the real LDA/GGA groundstate might differ from the configuration for which the potential was generated (most transition metals, see Sec. 10), since the occupancies have been set manually during the pseudopotential generation. For Pd, for instance, a $s^1d^9$ configuration was chosen to be the reference configuration, which is not the LDA/GGA groundstate of the atom. In this case, it is necessary to set the occupancies in VASP manually in order to obtain the same energy as the one found in the POTCAR file. This can be done including the following lines in the INCAR file:

  LDIAG = .FALSE.       ! keep ordering of eigenstates fixed
  ISMEAR = -2           ! keep occupancies fixed
  FERWE  =  5*0.9  0.5  ! set the occupancies manually

(5*0.9 is interpreted as 0.9 0.9 0.9 0.9 0.9). To determine the an initial WAVECAR file, it might be necessary to perform initial calculations using ICHARG=12 (i.e. fixed atomic charge density) and to continue with the setting above. After a successful atomic calculation compare the differences between the eigenvalues with those obtained by the pseudopotential generation program. The total energy written by VASP should be essentially zero (since the atomic reference energy EATOM is subtracted).

Another illustrative example: If the energy of an atom with a particular configuration needs to be calculated, i.e. spin polarized Fe with a valence configuration of 3d6.2 4s1.8, the calculation has to be done in two step. First a non selfconsistent calculation with the following INCAR must be performed:

 ISPIN = 2 
 ICHARG = 12
 MAGMOM = 4     ! magnetization in Fe is 4

This first step is required to determine a set of orbitals. From the OUTCAR file the level ordering can be determined:

 k-point   1 :       0.0000    0.0000    0.0000
  band No.  band energies    
      1         -5.0963      
      2         -5.0963      
      3         -5.0954      
      4         -5.0954      
      5         -5.0954      
      6         -4.6929      
      7         -0.7528      
      8         -0.7528      

 Spin component 2

 k-point   1 :       0.0000    0.0000    0.0000
  band No.  band energies     
      1         -3.6296       
      2         -2.2968       
      3         -2.2968       
      4         -2.2889       
      5         -2.2889       
      6         -2.2889       
      7         -0.1247       
      8         -0.1247

In the spin up component, the $5d$ states have lower energy than the s state, whereas in the down component, the $s$ state has a lower energy than the $d$ states (inspect PROCAR file). This ordering is important to supplying the occupancies in the lines FERWE and FERDO in the INCAR file in the second calculation. For a spherical atom, the final calculation is performed using the following INCAR file:

 ISTART = 1                 !  read in the WAVECAR file
 ISPIN = 2 
 MAGMOM = 4
 AMIX = 0.2 ; BMIX = 0.0001 !  recommended mixing for magnetic systems

 LDIAG = .FALSE.            !  keep ordering of eigenstates fixed
                            !  (Loewdin subspace rotation)
 ISMEAR = -2                !  keep occupancies fixed
 FERWE = 5*1  1*1     3*0   !  d5  s1, 3 other orbitals zero occ.
 FERDO = 0.8  5*0.24  3*0   !  s0.8 d1.2 other orbitals zero occ.

The determination of the spin-polarized broken symmetry groundstate of atoms is discussed in the next section 9.5.