[Pw_forum] What happens at REALLY large ectuwfc?

[Nicola Marzari]

Hi Brad,


probably the calculation is not converged.

Try a few things:

1) is the total energy at 2000Ry lower than at 200 Ry ? At 
self-consistency, it should be, even
infinitesimally, due to the variaitonal principle.

2) try different recipes: cg vs davidson, and different initalizations 
(atomic+random, random, etccc).

             nicola



Brad Malone wrote:
> I happened to stumble upon something today that I thought was 
> unusual.  The problem is illustrated in a simple example: if you take 
> the following basic cubic Si input file and, say, the Si.pz-vbc.UPF 
> pseudopotential from the QE website and do a test for convergence with 
> respect to ectutwfc.
>
> If we look at the 4 bands that are calculated we see the following 
> results:
>
> At ecutwfc=40 Ry:   -5.1740   7.0370   7.0370   7.0370
>     ecutwfc=100Ry:   -5.1746   7.0369   7.0369   7.0369
>     ecutwfc=200Ry:    -5.1746   7.0369   7.0369   7.0369
> Now try
>    ecutwfc=2000Ry:       -5.2862   6.9066   6.9066  10.2399
>
> Now why do the values change? If I look at the output file for the 
> 2000 Ry case I see that there is a negative starting charge:
>
> Initial potential from superposition of free atoms
>      Check: negative starting charge=   -0.057614
>
> But still, why is this? The usual answer for a negative starting 
> charge is to increase the wavefunction cutoff, although I suspect 
> that's not the problem in this case.....
>
> So besides the using of a 2000 Ry cutoff for silicon, what else is 
> wrong here?
>
> Best,
> Brad
> UC Berkeley
>
>
> &control
>    prefix = 'si'
>    calculation = 'scf'
>    restart_mode = 'from_scratch'
>    wf_collect = .false.
>    tstress = .true.
>    tprnfor = .true.
>    outdir = './'
>    wfcdir = './'
>    pseudo_dir = './'
> /
> &system
>    ibrav = 0
>    celldm(1) = 10.2612
>    nat = 2
>    ntyp = 1
>    nbnd = 4
>    ecutwfc = 2000.0
> /
> &electrons
>    electron_maxstep = 100
>    conv_thr = 1.0d-10
>    mixing_beta = 0.7
>    diago_full_acc = .true.
> /
> CELL_PARAMETERS cubic
>       0.000000000  0.500000000  0.500000000
>       0.500000000  0.000000000  0.500000000
>       0.500000000  0.500000000  0.000000000
> ATOMIC_SPECIES
>   Si  28.086  Si.pz-vbc.UPF
> ATOMIC_POSITIONS crystal
>   Si  -0.125000000  -0.125000000  -0.125000000
>   Si  0.125000000  0.125000000  0.125000000
> K_POINTS automatic
> 1 1 1 0 0 0
>
> ------------------------------------------------------------------------
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum
>   


-- 
---------------------------------------------------------------------
Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu