[CP2K-user] [CP2K:17101] Re: Density of states with k-points in CP2K

[Nicholas Winner]

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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