[Pw_forum] Work function calculation of CNT

[Nicola Marzari]

Dear Tuo-Hung,


most likely you are not looking at the electrostatic potential
in the vacuum region, i.e. away from the nanotube axis.

For a metallic nanotube, studied using pwscf, and treated as a metal
(i.e. with smearing and fractional occupations), the code at 
self-consistency will give you the fermi energy - this is calculated
so that the number of electrons between minus infinity and the fermi
energy is exactly the number of electrons of your CNT, taking into 
account the fact that, at finite temperature/broadenings, the occupation
function is not 1 below efermi and 0 above, but is smoothed out by the
thermal distribution.

Now that you have efermi, you need to know the vacuum level, i.e. the
energy of a test charge very far away from the tube:
the work function will be the difference between these two. PWSCF
does not set automatically this level to zero, and it would actually be
a fairly subtle exercise in electrostatics doing so.

So, what you need to do is to plot the electrostatic potential in your
unit cell, making sure that your unit cell is large enough so that your
potential has become flat very far away from the tube.

If your scf calculation is correct, the only thing that can go wrong is 
that you are not looking at the potential in the right places.

				nicola



Tuo-Hung Hou wrote:
> Hi!,
> 
> I am doing the work function calculation on carbon nanotubes. If I am 
> not mistaken, the scf calculation should give me Fermi energy or midgap 
> between HOMO and LUMO. Then, one can use pp.x plot_num 1 to generate the 
> total potential profile. The work function is decided by the difference 
> between the Fermi energy and the potential energy at the vaccum of the 
> supercell.
> 
>  
> 
> However, I am running into a problem that the Fermi energy from scf 
> (1.65 ev) is higher than any potential inside the entire supercell 
> (ranging from -10.4 to 0.4 eV).  When I looked at both the charge and 
> potential profiles, they seemed to suggest the Fermi energy should be 
> somewhere around -4 eV, which is not even close to the value scf 
> reported. I am very confused now. What is the Fermi energy I can use? 
> Any thoughts on this?

-- 
---------------------------------------------------------------------
Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu