binding energy

[xinhuanwen]

dear everyone!
grep 'TOTEN' OUTCAR
grep 'entropy' OUTCAR
EATOM=12.3

I have got
free energy TOTEN=-7.9100
energy without entropy=-5.8122


I want to know the binding energy.
binding energy=energy without entropy - EATOM=-5.8122-12.3=18.111

Is it right?
Or binding energy=-free energy TOTEN- EATOM=-7.910-12.3=-20.21

thanks

[forsdan]

First I must say I'm concerned about the energy difference between "free energy TOTEN" and "energy without entropy". Because this means that the entropy term in very large so maybe you should check your input files. Judging from the magnitude of the energies you're also calculating a small system. Guessing a molecule?

Note that for molecules and atoms you have discrete energy levels so the smearing should be nonexisting (or at least very small) and the ficticious entropy contribution should be very very small. The relevant energy for atoms and molecules should therefore be "energy without entropy".

To calculate the binding energy in a molcule you first run one calculation for the molecule and note the "energy without entropy" term and then you run calculations with the isolated atoms (one for each species) and extract the "energy without entropy" terms here as well. The binding energy is then the difference between these energies (energy of the molecule minus the sum of isolated atom energies). Please have a look at the section in the manual on how to calculate atom ground states.

Best regards,
/Dan Fors
<span class='smallblacktext'>[ Edited Sat May 17 2008, 10:04PM ]</span>