[CP2K-user] [CP2K:17109] Re: Density of states with k-points in CP2K

Matt Watkins mattwatkinsuk at gmail.com
Thu Jun 9 14:18:44 UTC 2022

edit - for DOS with k-points and MPI 

On Thursday, 9 June 2022 at 15:18:02 UTC+1 Matt Watkins wrote:

> 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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