[CP2K-user] [CP2K:17106] Re: Density of states with k-points in CP2K

Nicholas Winner nwinner at berkeley.edu
Thu Jun 9 13:55:32 UTC 2022

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.akira at gmail.com>

> 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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