cohesive energy of Silicon

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farzaneh · #1 Post by **farzaneh** » Tue Sep 07, 2010 7:11 am

I want to calculate the cohesive energy for Si. The output cohesive energy is much more than the experimental results (about 1 eV per atom). How can I calculate the cohesive enrgy accurately using VASP? Please give me guidance.

forsdan · #2 Post by **forsdan** » Tue Sep 07, 2010 12:35 pm

First of all: the energy in VASP is given with respect to the nonpolarized atom. To evaluate the cohesive energy you'll need to evaluate the spinpolarized state of the atom. Please see the manual:

http://cms.mpi.univie.ac.at/vasp/vasp/Atoms.html
http://cms.mpi.univie.ac.at/vasp/vasp/D ... atoms.html

Secondly, in general for the accuracy of the cohesive energy: Even if all your calculation settings (ENCUT, k-points, etc) are converged, please be aware of the impact of the exchange-correlation functional and its ability/limitation to accurately describe both the atom and the bulk phase.

Cheers,
/Dan

farzaneh · #3 Post by **farzaneh** » Tue Sep 07, 2010 2:51 pm

To consider the spin polarization effect, I set ISPIN=2, no difference in the energy. Is not this setting sufficient to calculate the spin polarization effect?

forsdan · #4 Post by **forsdan** » Tue Sep 07, 2010 4:43 pm

Let me rephrase my post into a question:

Have you performed a seperate calculation of a free Si atom in large box (~ 10x10x10 Ã…), where you have considered the spin-polarized broken symmetry of the atom according to the two links I gave?

A quick calculation for the Si atom with GGA/PBE gives me an energy about -0.8 eV, which then cancels most of the deviation (from the experimental value) you get for the cohesive energy. That is, if you havn't already accounted for this correction.

/Dan
<span class='smallblacktext'>[ Edited Tue Sep 07 2010, 06:54PM ]</span>

farzaneh · #5 Post by **farzaneh** » Tue Sep 07, 2010 6:06 pm

Thanks a lot,
Your comments solved my problem.

Zary · #6 Post by **Zary** » Mon Mar 31, 2014 12:16 pm

would you please explain me how run system for 1 free atom in a large box? Actually I wanna do the same calculation for Mn, I did the same as you said, but I received an error message!! For transition metals we need to consider a correction of energy, which is stated by VASP manual:
http://www.qft.iqfr.csic.es/docs/vasp/node241.html
Do you think I must only consider the correction or like what you said calculate energy for a free atom in a large box!??
Thanks in advance,