The POTCAR file:                                                                                                  

 

In order to tell VASP what atoms are in the POSCAR file, you need to create a POTCAR file.  The POTCAR file holds the pseudopotentials for all of the different atoms.  A pseudopotential is a modified expression for the potential that makes it possible to solve the Schrödinger equation.  (Google pseudopotential to see a visual of what this means). 

 

From the pseudopotential library, you create a POTCAR file that has the pseudopotentials for all of the atoms in your calculation.  For this calculation, we need the pseudopotential files for Mg and O.  The order of the pseudopotential files match the order you set in the POSCAR file. 

 

First copy the POTCAR files for each type of atom in your POSCAR to your working directory:

Copy the Mg pseudopotential:

cp /export/apps/vasp_pseudopotentials/paw/PBE/Mg_pv/POTCAR.Z .

 

Unzip the POTCAR.Z file:

gunzip POTCAR.Z

 

Rename the POTCAR:

mv POTCAR POTCAR.Mg_pv

 

Copy the O pseudopotential, unzip it and rename it:

cp /export/apps/vasp_pseudopotentials/paw/PBE/O_s/POTCAR.Z .

gunzip POTCAR.Z

mv POTCAR POTCAR.O_s

 

VASP only reads files called POTCAR (Not POTCAR.*).  You need to create one POTCAR file with both Mg and O in it:

 

cat POTCAR.Mg_pv POTCAR.O_s > POTCAR

 

Now check that both types are in the POTCAR:

grep TITEL POTCAR

 

And you should see:

         TITEL  = PAW_PBE Mg_pv 06Sep2000

         TITEL  = PAW_PBE O_s 07Sep2000

 

There is a lot of quantum mechanics in the POTCAR file that you will learn as time goes on.  For more details about using POTCAR.*_pv vs POTCAR.* files and soft vs. hard pseudopotentials, please see the wiki.

 

Reminder: The order of the pseudopotentials in VASP must MATCH the order of the atoms in the POSCAR file.