[Pw_forum] Hubbard U and small degauss ...

[Gabriele Sclauzero]

Dear Naol,

Naol Regassa wrote:
> Dear all
> 
> I have two questions to ask
> 
> a) For the last times I was calculating the effect of Hubbard U on the
> system YMnO_3 using QESPRESSO 4.10. I got an insulator band structure
> using U = 10.5eV though it is completely metal without U (GGA). When I

This is plausible. Though I don't know the physics of your compound, I can tell you that 
for simpler transition metal oxides (like FeO) you find the same change (i.e. from metal 
to insulator) after introducing the Hubbard U. Please have a look at Matteo Cococcioni's 
PhD thesis (from http://www.sissa.it/cm/phd.php) and the related paper (PRB 2005).

> got this result I have used small degauss = 0.006. My friend told me
> to use degauss  = 0.015. 

Have you asked him the reason? I don't see any...

In principle you should test convergence of the total energy (and other quantities you're 
interested in) w.r.t all the parameters which introduce significant approximations to your 
result. As you decrease degauss you'll reach the correct result for your system at T=0 
(chosing k-point grids accordingly), so I don't see any reason to choose higher degauss 
other that saving computer time.

In the specific case of DFT+U, you have to be carefull to converge the local occupations 
too. They are very sensitive to the value of ecutwfc, but also degauss and number k-points 
have to be checked.

Please tell me how possible (correct) is
> that? I checked the convergence for degauss but it is increasing near
> 0.015.

What is increasing?


> 
> b) Is adding U value more effective for the most stable structure ?

Sorry I cannot get this question... in which sense "more effective"?

> For the unstable states like for my case cubic ferromagnetic phase I
> can't see any change?

What are looking at? Maybe your question is clear to someone who's studying systems like 
yours, so it's because of me that I cannot get the point... but if you could be more 
precise maybe you would have more change to get help.


Regards,


GS



> 
> Here is the input for the 'scf'
> &control
>     calculation = 'scf',
>     restart_mode = 'from_scratch',
>     prefix = 'ymo_hcp_ferro'
>     tstress = .true.,
>     tprnfor = .true.,
>     pseudo_dir = '/home/MSC608/Desktop/QESPRESSO_PP',
>     outdir='/home/MSC608/Thesis/Hexagonal/Ferro-Hex/temp'
>  /
>  &system
>     ibrav = 0,
>     celldm(1) = 6.7,
>     nat =  10,
>     ntyp = 4,
>     ecutwfc = 32,
>     ecutrho = 340.0
>     nspin = 2
>     starting_magnetization(2) = 0.7,
>     starting_magnetization(3) = 0.7,
>     occupations = 'smearing',
>     smearing = 'marzari-vanderbilt',
>     degauss = 0.005,
>     lda_plus_u=.true.,
>     Hubbard_U(2) = 10.5,
>     Hubbard_U(3) = 10.5,
>  /
>  &electrons
>     diagonalization = 'david'
>     startingwfc = 'atomic+random'
>     mixing_beta = 0.3,
>     conv_thr =  1.0d-10
>  /
>  ATOMIC_SPECIES
>   Y     88.91     Y.pbe-nsp-van.UPF
>   Mn1   54.94     Mn.pbe-sp-van_mit.UPF
>   Mn2   54.94     Mn.pbe-sp-van_mit.UPF
>   O     15.9994   O.pbe-rrkjus.UPF
> 
> CELL_PARAMETERS (alat)
>    0.985236441  -0.266531554  -0.000316189
>   -0.266366323   0.985859979   0.000349871
>    0.000781869  -0.000890980   3.186836665
> 
> ATOMIC_POSITIONS (crystal)
> Y       -0.082663260  -0.082649328   0.000174846
> Y       -0.082874577  -0.082880779   0.499813069
> Mn1      0.205305857   0.560245090   0.249986248
> Mn2      0.561457263   0.207812583   0.750012603
> O       -0.027464907  -0.056329933   0.249979343
> O       -0.058482388  -0.031289340   0.750035308
> O        0.247951998   0.582250730   0.083800805
> O        0.582338704   0.248188482   0.916046842
> O        0.582135578   0.247948396   0.583964864
> O        0.247742343   0.582041831   0.416186070
>   K_POINTS automatic
>   3 3 2  1 1 1
> 
> Thanks alot in  advance
>  Tamene Regassa
>  AAU
>  Ethiopia
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> 

-- 


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| Gabriele Sclauzero, PhD Student                  |
| c/o:   SISSA & CNR-INFM Democritos,              |
|        via Beirut 2-4, 34014 Trieste (Italy)     |
| email: sclauzer at sissa.it                         |
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