[CP2K-user] [CP2K:17090] Re: Density of states with k-points in CP2K

[Nicholas Winner]

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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