[CP2K-user] Fermi energy too high?

Phil G. phil... at aol.com
Thu Jan 10 09:56:24 CET 2019


Dear Matt,

how can find the potential in the vacuum (which type of potential? 
potential energy or electric/electrostatic potential?) ?
For the case of electric/electrostatic potential, there is a flat curve 
with a step near the vacuum center as a consequence of dipole correction in 
Z direction, while in the bulk slab there is a periodic curve. 

I will try to use the wavelet solver with PERIODIC XY.

Phil

Am Mittwoch, 9. Januar 2019 14:05:04 UTC+1 schrieb Matt W:
>
> 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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