11.4 Atoms

 

About which files do you have to worry:

param.inc
INCAR
POSCAR
POTCAR
KPOINTS

Before using a pseudopotential intensively, it is not only necessary to check it in different bulk phases but the pseudopotential should also reproduce exactly the eigenvalues and the total energy of the free atom for which it was created. If the energy cutoff and the cell size are sufficient, the agreement between the atomic reference calculation (EATOM in the POTCAR file) and a calculation using VASP is normally better than 1 meV (but errors can be 10 meV for some transition metals). In most cases, calculations for an atom are relatively fast and unproblematic. For the calculation the should be used i.e. the KPOINTS file should have the following contents:

Monkhorst Pack
0
Monkhorst Pack
 1  1  1
 0  0  0

 

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 the dimer 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

Due to the large cell NGX... in the param.inc file must be increase accordingly; a setting with

      PARAMETER(NGX=40,NGY=40,NGZ=40,NGXF=64,NGYF=64,NGZF=64)

is sufficient in most cases (use the makeparam utility).

The INCAR file can be very simple

SYSTEM = Pd: atom
 Electronic minimization
   ENCUT  = 200.00 eV  energy cut-off for the calculation (opt)
   NELMDL =  5         make five delays till charge mixing

   ISMEAR =    0; SIGMA=0.1    use smearing method

The only difference to the bulk calculation is that Gaussian smearing should be used. If the atomic orbitals are almost degenerated, you might have to set SIGMA to a smaller value (but be careful very small values might degrade convergence significantly), for an initial test 0.1 eV is always a good starting point.

Mind: Look at the right value for the energy, it is not which contains a - meaningless - entropy term but but the ``energy without entropy'' in the OUTCAR file.

On the start of an atomic calculation it is often useful (but not required) to delay the charge mixing for a larger number of steps (in the example INCAR file above 5 steps) because the initial charge density corresponds already to the atomic charge density for which the pseudopotential was generated. It is also convenient to perform a calculation for a fixed atomic charge density (ICHARG=12) as a first test.

In some rare cases the real LDA groundstate might differ from the configuration for which the pseudopotential was generated (most transition metals, see Sec. 12). For the atomic all electron reference calculation the occupancies have been set by hand (for Pd a was chosen to be the reference configuration, which is not the LDA groundstate of the atom). In this case it is also necessary to set the occupancies for VASP also by hand, to get the same energy as that one found in the POTCAR file. This can be done including the following lines in the INCAR file:

  ISMEAR = -2
  FERWE  =  5*0.9  0.5

(5*0.9 is interpreted as 0.9 0.9 0.9 0.9 0.9). To find out the ordering of the eigenvalues it might be necessary to perform a calculation with ICHARG=12 (i.e. fixed atomic charge density). After a successful atomic calculation compare the differences between the eigenvalues with those obtained by the pseudopotential generation program. Also check the total energy, the differences should be smaller than 20 meV.

Mind: We have found that the size of the cell can be reduced if one special point is used instead of the point, i.e. if the KPOINTS file has the following contents:

Monkhorst Pack
0
Monkhorst Pack
 2  2  2
 0  0  0

The reasons for this behavior are: Due to the finite size of the cell a band dispersion exists i.e. the atomic eigenvalues split and form a band with finite width. To first order the center of the band lies exactly at the position of the atomic eigenvalues. Using the -point the eigenvalues at the bottom of the band are obtained. If the special point (0.25,0.25,0.25) 2 is used instead of the -point, the energy of the center of the band is obtained. Nevertheless we recommend this setting only for absolute experts: in most cases the degeneracy of the p- and d-orbitals is removed and only the mean value of the eigenvalues remains physically significant. In this cases it is also necessary to increase SIGMA or to set the partial occupancies by hand!