[Pw_forum] Workfunction and Magnetism

[Stefano Baroni]

On Jul 20, 2004, at 5:41 AM, Aloysius Soon Sze Lok wrote:

> Dear all,
> I have 2 questions I would like to consult:
>
> 1.	After calculating surface energy for a slab calculation, can we use 
> PWSCF
> to extract the workfunction of the surface? If so, is it specify 
> somewhere?
> I'm interested in comparing the change in the workfunction between the 
> clean
> surface and the adsorbate-surface.

What is the work function?
- The minimim work (energy) to extract an electron from a solid

work = (final energy) - (initial energy)
in order to minimize a difference, you have to minimize the first term 
and maximize the second

What is the maximum initial energy that an electron being ionized can 
have?
- The Fermi energy (provided by the code)

What is the minimum final energy that an electron being ionized can 
have?
- The "vacuum level" i.e. the value that the potential felt by an 
electron  (the Kohn-Sham potential, also calculated and used by the 
code) has far from the surface.

How far?
- The farthest (i.e. in the middle of the vacuum region in between two 
adjacent slabs)

Beware:
* The Kohn-Sham potential and the Hartree potential converge to the 
same value far from the origin; the second converges faster. You see 
why?
* A good estimate is not necessarily the best that you can extract from 
a given calculation. Convergence has always to be checked. i.e. you may 
want to plot the potential (or rather the planar average of it parallel 
to the surface) as a function of the the distance from the surface, and 
see if and to which value it converges ... In other words, the vacuum 
region may be thick enough for calculating the surface energy, but not 
so for the work function ...

> 2.	As I will be working on magnetic systems very soon, I would like to 
> ask how
> to specify it in the run. I understand that some of the parameters 
> like nspin
> will be set to 2 and the starting_magnetization(1) will be needed to 
> be set
> between -1 and 1. So what would the values 1 and -1 mean? I will be 
> working
> specifically on iron sulphide systems.

most probably, (rho_up-rho_down)/(rho_up+rho_down)

Hope this helps
Stefano Baroni

---
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