[CP2K-user] Fermi energy too high?

[Matt W]

Is your slab in the centre of the cell (Y direction)? -  the cell runs from 
0 to L, so the slab must be centred at L/2.

Matt

On Tuesday, January 15, 2019 at 2:21:26 PM UTC, Phil G. wrote:
>
> Dear Matt,
>
> thank you for the suggestions and after centering the slab in y-direction 
> and turning off the surface dipole correction, the program finally runs and 
> I get the result, but there are some error messages such as:
>
>  *** WARNING in pw/ps_wavelet_methods.F:236 :: Density non-zero on the ***
>  *** edges of the unit cell: wrong results in WAVELET solver           ***
>
>     89 Broy./Diag. 0.10E+00    2.3     0.00000880     -3020.8753805067  
> 4.85E-05
>
>   *** SCF run converged in    89 steps ***
>
> I have chosen the cell length of 40 angstroms in Y direction (slab length 
> in y-direction is about 27.6 angstroms).
> For the LiNbO3 slab consisting of 14 trilayers as stated in the message of 
> 9th January, I obtain the result of the Fermi energy:
>
>   E_F = 11.174 eV    (in comparison to the -0.8516 eV in case b) )
>
> This is unrealistic...should I have to enlarge the cell length in 
> y-direction or should I turn on the dipole correction?
>
> Phil
>
>
>
> Am Dienstag, 15. Januar 2019 12:31:22 UTC+1 schrieb Matt W:
>>
>> Ah, OK. The extended FFT lengths only works with FFTW not with the 
>> wavelet FFT.
>>
>> You do not need such a large vacuum with the wavelet solver as it is 
>> genuinely non-periodic. Place the slab in the center and 5A of vacuum 
>> either side should be sufficient - allow 10 A either side to get a clear 
>> decay to vacuum level(s). You will get two vacuum levels if you have  a 
>> dipole. I don't know if wavelet will work with the dipole correction, I'd 
>> turn it off to start with.
>>
>> Matt
>>
>> On Tuesday, January 15, 2019 at 8:52:35 AM UTC, Phil G. wrote:
>>>
>>> Hello again,
>>>
>>> have tried some attempts to start calculation with WAVELET poisson 
>>> solver, but all attempts have failed due to following error messages:
>>>
>>> 1)  the FFT in the x direction is not allowed
>>>      n01 dimension         154
>>>      (pw/ps_wavelet_util.F:358)
>>>
>>>      ===== Routine Calling Stack ===== 
>>>
>>>            13 S_FFT_dimensions
>>>            12 RS_z_slice_distribution
>>>            11 ps_wavelet_create
>>>      the FFT in the x direction is not allowed
>>>      n01 dimension         154
>>>            10 pw_poisson_rebuild
>>>             9 pw_poisson_solve
>>>             8 qs_ks_build_kohn_sham_matrix
>>>             7 rebuild_ks_matrix
>>>      the FFT in the x direction is not allowed
>>>      n01 dimension         154
>>>      the FFT in the x direction is not allowed
>>>      n01 dimension         154
>>>      the FFT in the x direction is not allowed
>>>      n01 dimension         154
>>>      the FFT in the x direction is not allowed
>>>      n01 dimension         154
>>>             6 qs_ks_update_qs_env
>>>             5 scf_env_do_scf_inner_loop
>>>             4 scf_env_do_scf
>>>             3 qs_energies
>>>             2 qs_forces
>>>             1 CP2K
>>>
>>>
>>> 2)  after that I turn off the command EXTENDED_FFT_LENGTHS, then:
>>>
>>>     Index to radix array not found.
>>>     (pw/fft_tools.F:293)
>>>
>>>      ===== Routine Calling Stack ===== 
>>>
>>>             6 pw_grid_setup
>>>             5 pw_env_rebuild
>>>             4 qs_env_rebuild_pw_env
>>>             3 qs_env_setup
>>>             2 qs_init_subsys
>>>             1 CP2K
>>>
>>>
>>> That's strange and I don't know what to do. 
>>> In my input file there are some info about commands:
>>>
>>> [...]
>>>     SURFACE_DIPOLE_CORRECTION .TRUE.
>>>     SURF_DIP_DIR Y
>>> [...]
>>>     &MGRID
>>>       CUTOFF 600
>>>       NGRIDS 5
>>>       REL_CUTOFF 50
>>>     &END MGRID
>>> [...]
>>>     &POISSON
>>>       POISSON_SOLVER WAVELET
>>>       PERIODIC XZ
>>>     &END POISSON
>>> [...]
>>>
>>> [...]
>>>     &CELL
>>>       A    5.148      0.0         0.0
>>>       B    0.000      100.0      0.0
>>>       C    0.0          0.0         8.9166         
>>>       PERIODIC XZ
>>>     &END CELL
>>> [...]
>>>
>>> Phil
>>>
>>>
>>> Am Donnerstag, 10. Januar 2019 10:56:24 UTC+1 schrieb Phil G.:
>>>>
>>>> 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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