[Pw_forum] What happens at REALLY large ectuwfc?
Nicola Marzari
marzari at MIT.EDU
Sat Jan 23 12:49:25 CET 2010
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
>
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Prof Nicola Marzari Department of Materials Science and Engineering
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tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu
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