[CP2K-user] Fermi energy too high?

[Phil G.]

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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