Eigenvalue shifting - alpha+bet term

Message

cdt9f · #1 Post by **cdt9f** » Fri Aug 26, 2005 7:18 pm

Hi there,

I typically adjust my Fermi level according to the vacuum level of the electrostatic potential to get my work function and also to place DOS plots on an 'absolute' scale.

Recently I was recommended to use the alpha+bet value which is listed with the E-fermi instead.

Is this approach sound? And how is the alpha+bet value derived? I tried looking at the documentation and workshop pdf's but have not found where this is explained.

Thanks,
Chris

#2 Post by **admin** » Tue Aug 30, 2005 3:25 pm

You need to determine the vacuum potential to find the absolute meaning of the eigenvalues!
Adding alpha+beta is not very accurate and gives the correct answer only in the limit of very large supercells.

In any case the absolute value of the eigenvalues can be determined only if your cell contains sufficient vacuum. For a slab, the energy difference between the topost eigenvalue (Fermi-level) and the vacuum potential, corresponds to the workfunction (for bulk calculation the absolute eigenvalues are in principle not well defined).
To get the vacuum potential, you need to write the LOCPOT file and you need to inspect it visually (at positions that are far from the slab).
The program vtotav.f might aid this. It averages the LOCPOT file along specified planes.

cdt9f · #3 Post by **cdt9f** » Wed Aug 31, 2005 2:19 am

OK, I have used the procedure you suggest for some time. However, to resolve some controversy with colleagues, I would still like to know what alpha+beta means.

Chris

aron · #4 Post by **aron** » Fri Sep 28, 2007 2:13 pm

[quote="cdt9f"] I would still like to know what alpha+beta means.[/quote]

I'm curious about this too. Is it related the ionization potentials of the isolated atoms?

Modey3 · #5 Post by **Modey3** » Wed Jul 22, 2009 9:31 pm

"I would still like to know what alpha+beta means."

This question needs to be answered. If anybody has any info please advise. Thanks

Best Regards

modey3

asija · #6 Post by **asija** » Wed Jul 14, 2010 11:16 am

oh, thats a terrible work. If I do relaxed scan where I have arround 100 different geometries it's almost impossible to write LOCPOT, look inside, find appropriate point in space, note the potential value and substract it manually for each of 100 geometries

( I really need something which can be automated (by script or program).