[Pw_forum] vc-relax with diverging SCF error

[Stefano de Gironcoli]

I think that 22 Ry may be too small for Cu, even with USPP, especially 
if stress is calculated.
try to reduce the mixing_beta.

stefano


Christopher O'Brien wrote:
> PWSCF users:
> I have been struggling for quite some time to perform a vc-relax calculation on a simple system of 96 Cu atoms. I am confident that the input is correct as an ion relaxation run converges in one scf cycle to a pressure of -11.54 kbar with the same parameters as the vc-relax calculation (shown below). 
>
> As seen in the output file, the problem with the vc-relax calculation is that the SCF error will increase with each iteration. I have tried to remedy this issue by restricting the maximum movement distance of both the cell and ions to no avail. Unfortunately, I cannot turn on symmetry as I am using this as a test case for further calculations of Al-Cu solid solutions. 
>
> Is there anything terribly wrong with my input? Should I impose symmetry despite the addition of Al to the Cu lattice? 
>
> Thanks for your help.
>
>   
> ------------------------------------------------------------------------
>
>
>
>  &CONTROL
>                  calculation = 'vc-relax' ,
>                 restart_mode = ‘from_scratch’ ,
>                   wf_collect = .false. ,
>                       outdir = './' ,
>                   pseudo_dir = './' ,
>                       prefix = 'Cu' ,
>                       forc_conv_thr = 2.0d-2,
>                       etot_conv_thr = 1.0d-3,
>                      tstress = .true. ,
>                    verbosity = 'minimal' ,
>                    disk_io = 'none' ,
>                      tprnfor = .true. ,
>  /
>  &SYSTEM
>                        ibrav = 8,
>                    celldm(1) = 9.505756,      
>                    celldm(2) = 1.735408,
>                    celldm(3) = 4.921944,
>                          nat = 96,
>                         ntyp = 1,
>                      ecutwfc = 22 ,
>                      ecutrho = 220 ,
>                        nosym = .true. ,
>                  occupations = 'smearing' ,
>                      degauss = 0.022 ,
>                     smearing = 'methfessel-paxton' ,
>  /
>  &ELECTRONS
>                     conv_thr = 1.0d-5 ,
>                  mixing_beta = 0.7 ,
>  /
>  &IONS
>                 ion_dynamics = 'bfgs' ,
>  /
>  &CELL
>                        press = 0.0 ,
>               press_conv_thr = 5.0d0,
>                  cell_dofree = 'xyz' ,
>                cell_dynamics = 'bfgs' ,
>  /
> ATOMIC_SPECIES
>    Cu   63.54600  Cu.pw91-n-van_ak.UPF 
> ATOMIC_POSITIONS (crystal)
> Cu       0.000000000  -0.000082163  -0.000018624
> ...
> K_POINTS automatic 
> 6 4 2 1 1 1 1 
>
> ===================================================================
> Christopher J. O'Brien
> cjobrien at ncsu.edu
> https://sites.google.com/a/ncsu.edu/cjobrien/
> See what I'm citing at http://www.citeulike.org/cjo123
>
> Ph.D. Candidate
> Computational Materials Group
> Department of Materials Science & Engineering
> North Carolina State University
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