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

[Johannes Meusburger - STFC UKRI]

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).
 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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