[CP2K-user] [CP2K:17105] Re: Density of states with k-points in CP2K

[Marcella Iannuzzi]

For any delta_e do you get all zeros?
And if delta_e = 0?

About the settings, they look fine to me. 
Why are you using a Fermi-Dirac smearing for GaAs, it is not a metal. 
Anyway, this should not change anything with respect to the energy gap.
Have you considered to run the same calculation with multiple cells instead 
of k-points?  

Regards
Marcella

On Wednesday, June 8, 2022 at 10:49:31 PM UTC+2 nwi... at berkeley.edu wrote:

> Marcella, I looked further and played with delta_e and there is something 
> that seems incorrect to me.
>
> If I let delta_e -> 0 then the density column is equal to 0 everywhere. It 
> looks like this might be due to precision? The column only lists 3 decimal 
> places.
>
> On Monday, June 6, 2022 at 1:02:40 PM UTC-7 Nicholas Winner wrote:
>
>> Thank you Marcella. I am clear about the Fermi level setting now. 
>>
>> Any ideas as to why calculated gap is so large (first image)? The input 
>> file I have attached has pretty reasonable parameters I think:
>>
>>    - multigrain can accommodate all basis functions
>>    - precision settings are above average 
>>    - TZVP basis sets
>>    - Tight MP k-point grid of 16x16x16
>>    - EPS_SCF of 1e-6
>>
>> Not sure what else could be causing such a large discrepancy without 
>> published PBE results. Is it still an issue of interpreting the DOS?
>> On Friday, June 3, 2022 at 2:38:05 AM UTC-7 Marcella Iannuzzi wrote:
>>
>>>
>>>
>>> Hi, 
>>>
>>> The Fermi energy is not well defined for electronic structures with an 
>>> energy gap. 
>>> Using the Fermi-Dirac distribution, CP2K sets the Fermi energy somewhere 
>>> in the middle of the gap.
>>>
>>>
>>> The histogram calculated by activating PRINT%DOS is just counting the 
>>> number of eigenvalues falling into each histogram bit.
>>> The partitioning of the histogram in bits is determined from the DELTA_E 
>>> keyword, the default of this is 0.001.
>>> The final value of the density (sum of states per bit) is then 
>>> normalised by the number of NMO.  
>>> The column occupation reports the sum of the occupation numbers of the 
>>> eigenstates with energy falling into the corresponding bit.
>>> For small DELTA_E, there are bits with zero density and zero occupation 
>>> number. 
>>> Setting DELTA_E to zero, the histogram is replaced by simply the list of 
>>> eigenvalues. 
>>>
>>> When using k-point sampling the procedure is more or less the same, but 
>>> each contribution to the density as well as each contribution to the 
>>> occupation are multiplied by the weight of the corresponding k-point. 
>>>
>>> Regards
>>> Marcella
>>>
>>>
>>>
>>> On Thursday, June 2, 2022 at 10:05:09 PM UTC+2 nwi... at berkeley.edu 
>>> wrote:
>>>
>>>> It is a static calculation, so iteration = 0. I redid the calculation 
>>>> with cell optimization. Didn't seem much of change. Here at the plots using 
>>>> the occupation first and the density second. I set fermi level to be 0, and 
>>>> it doesn't put it anywhere near the VBM like we expect. In the second case 
>>>> it looks like there is a gap, but its too big. 
>>>>
>>>> Any knowledge maybe about the definition of fermi level? Maybe that's 
>>>> where I'm tripping up.
>>>> [image: Screen Shot 2022-06-02 at 1.00.02 PM.png]
>>>> Do you know anything about the fermi level definition?
>>>> [image: Screen Shot 2022-06-02 at 12.59.51 PM.png]
>>>> On Thursday, June 2, 2022 at 9:04:42 AM UTC-7 wave... at gmail.com wrote:
>>>>
>>>>> Hi
>>>>>
>>>>> It's not clear to me why you are looking at the first step. Did it 
>>>>> converge in just one step?
>>>>>
>>>>> At any rate, you'll notice a difference between &DOS and &PDOS. PDOS 
>>>>> returns values per orbital (s, p, d, f). If I understand it correctly, the 
>>>>> density is the DOS as a solid state physicist would understand it, and 
>>>>> occupation gives you a fractional filling at that energy. Not quite the 
>>>>> same thing.
>>>>>
>>>>> Regards
>>>>> Sam
>>>>>
>>>>> On Wednesday, June 1, 2022 at 7:39:20 PM UTC+2 nwi... at berkeley.edu 
>>>>> wrote:
>>>>>
>>>>>> Hi everyone, 
>>>>>>
>>>>>> I know that DOS and points aren't what people first think of when 
>>>>>> using cp2k, but the functionality does seem to be there in some limited 
>>>>>> way... 
>>>>>>
>>>>>> Anyway, I'm wondering how to properly use it, because my test case 
>>>>>> doesn't look right. I'm running a point calculation on GaAs with a 16x16x16 
>>>>>> Monkhorst-Pack pack grid and then printing the total DOS. I then take the 
>>>>>> columns from the DOS file (head attached) and plot energy (column 1) 
>>>>>> against occupation (column 3) using the reported Fermi energy from the 
>>>>>> output file of ~0.14 Ha. This results in a DOS where the Fermi energy does 
>>>>>> not line up with the VBM. What's more, if I visually inspect this DOS, the 
>>>>>> band gap of GaAs is >1eV. While that is closer to the experimental value, 
>>>>>> the reported band gap using GGA PBE is closer to 0.3eV.  
>>>>>>
>>>>>> Questions:
>>>>>> (1) What is the distinction between columns 2 and 3 in the DOS file? 
>>>>>> They give similar, but not equivalent looking dos plots.
>>>>>> (2) Is the Fermi energy printed in the output file the same as the 
>>>>>> Fermi energy of the DOS, or is it defined different?
>>>>>>
>>>>>> I've attached my input file for reference. Thanks in advance.
>>>>>>
>>>>>>  # DOS at iteration step i = 0
>>>>>>
>>>>>>     Energy[a.u.]       Density     Occupation
>>>>>>
>>>>>>     -0.40682067         0.0009         0.0957
>>>>>>
>>>>>>     -0.40582067         0.0015         0.1543
>>>>>>
>>>>>>     -0.40482067         0.0000         0.0000
>>>>>>
>>>>>>     -0.40382067         0.0000         0.0000
>>>>>>
>>>>>>     -0.40282067         0.0034         0.3516
>>>>>>
>>>>>>     -0.40182067         0.0026         0.2656
>>>>>>
>>>>>>     -0.40082067         0.0038         0.3828
>>>>>>
>>>>>>     -0.39982067         0.0000         0.0000
>>>>>>
>>>>>> [image: Screen Shot 2022-06-01 at 10.32.10 AM.png]
>>>>>>
>>>>>

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