A few questions regarding units and such

[panda]

Hoping someone can help me!

1.) What are the units of charge density? I know charge/unit volume, but what is the unit volume from the CHGCAR file? Values in CHGCAR range from 0.850*10^3 to 0.11*10^-8 (roughly 850 - 0) but this can't be electrons/A^3, that doesn't make any sense.

2.) Forces - in the OUTCAR the forces are reported in eV, but this unit doesn't make sense for a force. What are the units of force?

3.) Free Energy TOTEN - how is it calculated? From vibrational frequencies? Is it a true Free Energy?

~~~Thanks In Advance!!!~~~
<span class='smallblacktext'>[ Edited ]</span>

[forsdan]

1. The values given in the CHGCAR file are the charge density multiplied with the volume of the cell. This is clearly stated in the manual:

http://cms.mpi.univie.ac.at/vasp/vasp/CHGCAR_file.html

2. The forces are given in eV/Angstrom

3. For instance for metals a smearing scheme is used to facilitate the integration over Brillouin zone. The procedure involves a fictitious smearing of the electronic states, which gives rise to an entropy contribution. This is a purely electronic contribution (NOT a vibrational contribution). If the smearing width is sufficiently small an accurate interpolation to zero smearing is then possible.

So the entropy contribution (and hence the given free energy) is fictitious and is mainly a computational mean. To what extend you then can go on and use it as an approximation for the electronic free energy contribution if you want to treat finite temperatures (T > 0 K) is another issue.

Cheers,
/Dan

<span class='smallblacktext'>[ Edited Tue Jul 06 2010, 11:36PM ]</span>

[panda]

Dan,

I have read the CHGCAR section of the manual, just didn't really understand it fully. I was pretty sure vibrational frequencies were not used but just wanted to make sure - I will have to think about to what extent I can apply the finite temperature approximation. Thanks so much for your reply, it was very helpful!