[CP2K-user] Fermi energy too high?

Matt W mattwa... at gmail.com
Wed Jan 16 13:20:18 CET 2019


I added

&CENTER_COORDS
&END

in the topology section. I get the 'fermi level' reported at -4.61 eV. And 
the vacuum levels at +- 0.75 eV

So the work functions of the two faces are ~5.4 and 3.8 eV respectively. I 
got the profile of the potential by adding &V_HARTREE_CUBE and using the 
cubecruncher utility (bundled in the tools directory) to extract the 
averaged y profile to find the vacuum levels.

Matt



On Wednesday, January 16, 2019 at 9:26:32 AM UTC, Matt W wrote:
>
> Hi,
>
> the xyz file you sent doesn't have the y coords centred in the cell. The 
> cell goes from 0 to L. You centre as if it goes from -L/2 to L/2.
>
> If you print V_HARTREE_CUBE you can work out the potential profile across 
> the slab. 
>
> BTW your slab is too small in X and Z for reliable results without k-point 
> sampling...
>
> Matt
>
> On Wednesday, January 16, 2019 at 8:54:56 AM UTC, Phil G. wrote:
>>
>> 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
>>>>>>>>>>>
>>>>>>>>>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.cp2k.org/archives/cp2k-user/attachments/20190116/56a7c106/attachment.html>


More information about the CP2K-user mailing list