In VASP, PAW potentials are supposed to reproduce the total energy of the free atom (EATOM)
for which they were created. However, for Se, for example, I found that the energy of Se "in a box" is always lower than EATOM ( by ~0.272 eV). This difference practically does not change if the energy cutoff and the box size are increased. Why?
I don't see such a large difference in my calculations. For Se, I just did a test (10 Å box) and the atomic energy EATOM I get is 257.1266 which is just 2 meV (0.002 eV) less than the corresponding value in the PAW pseudopotential (EATOM=257.1286). You need to make sure you use the same functional as that written in the pseudopotential file (in my case GGA=PE).
Ivan,
The EATOM is the energy of individual atoms in significatly large boxes at reference point. If you evaluated the energy of your atom in a box with periodic boundaries, the interations with neighboring atoms would affect the energy value.
Dear Alireza Asiaee
The value of EATOM is independent of the box size. This is basically the atomic energy of each ion as a single entity. You can easily test this for any atom with different box sizes and will see the value remains the same.
Of course the EATOM is independent of box size. My interpretation from Ivan question is whay EATOM energy value is different from the energy of the same atom in a box, where we have the interaction of neighboring atoms from priodic effects.
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