[CP2K-user] Fermi energy too high?

Matt W mattwa... at gmail.com
Wed Jan 9 13:05:04 CET 2019


Hello again,

did you find the potential in the vacuum and align to that? You need to set 
a reference to get absolute values.

You could also try using the wavelet solver 

&POISSON
   PSOLVER WAVELET
   PERIODIC XZ
&END

and PERIODIC XZ  in the &CELL section. The Y direction must be the 
non-periodic one. That gives an absolute reference (if there is no dipole 
in the cell otherwise you need the  dipole correction switched on).

Matt

On Wednesday, January 9, 2019 at 8:18:58 AM UTC, Phil G. wrote:
>
> Dear Matt,
>
> thank you for your reply and good suggestions. Now I have let different 
> LiNbO3 slab systems to be calculated:
>
> a) 14 trilayer system as from Sanna et al., *Appl. Surf. Sci.* *301* 
> (2014), 70-78 with Nb-O3-Li2 surface termination on the one side of the 
> slab and Li-O surface termination on the other side. Vacuum space of at 
> least 40 Angstroms was included. The bulk region was already 
> geometry-optimized and bulk atoms were fixed in the inner 6 trilayers. 
> Geometry optimization on the whole slab system was performed and then the 
> pdos of the system was calculated and plotted for every atom layers.
> Result: E_F = 0.1552 eV  (fermi energy is overall constant, in every atom 
> layers)
>
> b) the same as a), but the bulk region was not already geometry-optimized 
> before. Geometry optimization was performed and calculation of pdos.
> Result: E_F = - 0.8516 eV
>
> c) the same as b), but 26 trilayers instead of 14 trilayers. Geometry 
> optimization and calculation of pdos were performed.
> Result: E_F = 2.3372 eV
>
>
> So, I am wondering why these values differ so much. Should I need band 
> structure calculation of the bulk LiNbO3 in order to find the global 
> valence band edge maximum (with KPOINT calculation)?
>
> Kind regards,
>
> Phil
>
>
> Am Freitag, 14. Dezember 2018 17:41:03 UTC+1 schrieb Matt W:
>>
>> In a periodic system the zero of the one electron levels is arbitrary. If 
>> you need a reference you need to run a slab calculation with vacuum or try 
>> to align semi-core states to something.
>>
>> Matt
>>
>> On Friday, December 14, 2018 at 4:33:13 PM UTC, Phil G. wrote:
>>>
>>> Dear people and experts of CP2K,
>>>
>>> after the geometry optimization of the lithium niobate (LiNbO3) unit 
>>> cell I would like to obtain pdos in order to determine the band gap and 
>>> Fermi energy of the bulk system.
>>> After the calculation with ENERGY_FORCE I got pdos files of the three 
>>> atoms (indexing depends on the z-position of the atoms) and I'm wondering 
>>> about the value of Fermi energy: E_F = 0.300174 a.u. which is 8.168 eV. Is 
>>> that not too high? And which energy has the value 0 and what is the 
>>> reference? What is the Fermi energy defined in the language of CP2K?
>>> The energy band gap (HOMO-LUMO gap) of 3.62 eV agrees well with 
>>> experimental values of 3.7 to 3.9 eV. But I cannot imagine that Fermi level 
>>> has too high energy values.
>>>
>>> Has anyone an idea what is the reason for such high Fermi energy values?
>>>
>>> Here the input and output files are attached here.
>>>
>>> Kind regards,
>>>
>>> Phil
>>>
>>
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