[CP2K-user] Fermi energy too high?

Phil G. phil... at aol.com
Wed Jan 16 08:54:56 CET 2019


yes, the slab was centered in y-direction (xyz file attached here), but in 
x- and z- direction from 0 to the corresponding lengths of the slab.
Maybe I should extend the size of vacuum space or turn on the dipole 
correction?
For searching for reason, the input file is also attached here.

Phil

Am Dienstag, 15. Januar 2019 16:07:55 UTC+1 schrieb 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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