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I calculated the binding energy of Cl2 using PAW_GGA. The calculated binding energy is about 2.96 eV (3.50eV for Cl2 total energy and 0.29 eV for Cl total energy), while the experimental value is about 2.5 eV, the dofference is about 0.4 eV. Could this difference be that large in the vasp calculation? What could be the reason for this large difference?


SYSTEM = Cl2 molecule

ISMEAR = 0
SIGMA = 0.00001
IALGO = 48
LPLANE=.TRUE.
NPAR = 1
NSIM = 1

PREC = Accurate
NELMIN = 6
EDIFF = 1E-6
EDIFFG = -0.01
NSW = 500
IBRION = 2

and

SYSTEM = Cl atom

ISMEAR = 1
SIGMA = 0.00001
IALGO = 48
LPLANE=.TRUE.
#NPAR = 2
#NSIM = 2

NELMIN = 6
EDIFF = 1E-6
EDIFFG = -0.01
NSW = 500
IBRION = 2
ISPIN=2

This may be due to the discrepancy of the functional.

Check convergence of your result with respect to:
- size of the cell
- value of ENCUT, ENAUG, PREC, EDIFFG (= -0.001 maybe?)
- try different PAW Potentials

I think ISMEAR = 0 and SIGMA = 0.01 or 0.001 could be safer.
The remaining error is most likely due to the functional.
PAW vs AE-Potential could also be responsible for some of the error.

for small molecules, there may be significant discrepancy in the bonding energies of small molecules, depending on the chosen XC-functional (please also note that the DFT calculations correspond to T=0K). usually the PBE (3.04) or rPBE (2.7) potentials give better results. These values are taken from the PhD-Thesis of Robin Hirschl. (R. Hirschl, Thesis, University of Vienna, 2002)

I used the same INCAR the first Guy put here. Here is my result
PBE=2.84eV, GGA= 3.08eV, LDA=3.54eV. These valuse are far from expermental value=2.5eV. It is simple due to the system error of XC function or something else?

Best result can be obtained by using GGA+HSE06.

I calculated the binding energy of Cl2 as 2.59 eV.