[CP2K-user] [CP2K:17108] Re: Density of states with k-points in CP2K

[Matt Watkins]

Think there might be a bug fix for k-points and MPI processes in the works.

On Thursday, 9 June 2022 at 14:55:48 UTC+1 nwi... at berkeley.edu wrote:

> I do not get zeros for all delta_e, only when it is below a certain 
> threshold. When I take the default delta_e, it gives an energy gap of ~1.5. 
> When I increase delta_e to .01 the band gap shrinks to ~1. So there is some 
> kind of a binning problem it seems.
>
> I was using Fermi Dirac smearing simply to get CP2K to print out the fermi 
> energy, which it won't do in k-point calculation without it. I could just 
> read off the VBM from the list of eigenvalues, but I try to automate this.
>
> I'm hoping to do this as a kpoint calculation so that I can get a good 
> DOS. While the energy gap is possible with gamma point only, the DOS isn't 
> really. Generally CP2K is used as a gamma-only code for large supercells, 
> but I hope to use it as a slightly more general solver since I like the 
> code and it seems to have most of the functionality needed.
>
> On Thu, Jun 9, 2022 at 1:28 AM Marcella Iannuzzi <marci... at gmail.com> 
> wrote:
>
>> 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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>

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