[QE-users] convergence testing very high dual parameter ecutrho for ultrasoft pseudopotential

[Giovanni Cantele]

Dear Johannes,

I don’t know if I’ve well understood your point or if I’m missing something. However, after looking yo the plots you posted on Google Drive, it seems to me that actually your forces are fully converged even at the smallest values of ecutrho.
For example, in S.tif, I see the scale ranging from 0.130328 to 0.130340 Ry/a.u., about 1.e-5 wide. Similarly, just to consider another case, in O5.tif forces range from 0.45811 to 0.45815, about 4.e-5 wide (and if you discard the first point the range is reduced by a further factor of 10).

Now, the default value for convergence of forces is 1.e-3 a.u., see https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm119 <https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm119>

So, this means that your forces are much more converged than the threshold used by QE, unless you otherwise specified in your input.

You can also see some sample convergence plots on forces here:
http://theossrv1.epfl.ch/Main/ElectronicTemperature <http://theossrv1.epfl.ch/Main/ElectronicTemperature>
The vertical axis spans a range of about 0.2 eV / A ~ 0.2 / 25.7 Ry / Bohr ~ 0.008 Ry / Bohr, again ~2-3 orders of magnitude larger that the same range in your plots.

Giovanni

> On 1 Mar 2022, at 19:00, Johannes Meusburger - STFC UKRI via users <users at lists.quantum-espresso.org> wrote:
> 
> Dear all, 
> I have observed a rather curious behaviour when performing convergence testing for CuSO4 x 5H2O. For the convergence testing I have displaced the oxygens surrounding the Cu and S atom by 10% of the equilibrium Cu-O and S-O bond lengths and computed the forces on each of the atoms for various k-point grids, smearing widths (marzari-vanderbilt type), ecutrho and ecutwfc. To my surprise I found that the ecutrho value did not converge up to a dual parameter of 5 (i.e., wfc cut = 50 Ry and ecutrho = 1000 Ry; forces vs dual parameter plots available on google drive:https://drive.google.com/drive/folders/1YaotezLaEHiSKouW19azGMdC6T8J-Jg8?usp=sharing <https://drive.google.com/drive/folders/1YaotezLaEHiSKouW19azGMdC6T8J-Jg8?usp=sharing>). 
>  I am using ultrasoft PP from the GBRV library, usually I find that a dual parameter of 2 or 3 (i.e., ecutrho 8 or 12 times larger than ecutwfc) yields well converged forces. My question now is if there is any reason, why I should NOT use such a high dual parameter for the geometry optimization? 
>  
> All the best and many thanks, 
>  
> Johannes 
>  
> Johannes Meusburger  
> ISIS Neutron and Muon Source, UK 
>  
> 
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