[Pw_forum] cell optimization

[Stefano Baroni]

I may be wrong, but your problem could be due to subtelties of the US  
PP formalism. Using US PPs, I am afraid that PW calculations are  
variational with respect to PW expansion coefficients only when the  
augmentation charge is at convergence. Convergence with respect to the  
augmentation charge usually is a rather delicate issue. For sure, the  
cutoff for the charge-density distribution (ecutrho) should be taken  
larger (sometimes much larger) than the default value for norm- 
conserving PPs (4 x ecutwfc). In yor calculation, you seem to use just  
this value, which may turn out to be inadequate. Have you tried to  
test the convergence of your calcs wrt echutrho (keeping ecutwfc  
fixed)? SB

On Jun 22, 2008, at 11:06 AM, hania djani-ait aissa wrote:

> Dear Nicola,
> I have tried startingwfc='random' and it gave me the same energies  
> values as for startingwfc='atomic' with still the same problem of  
> drop and rise of total energy. I also increase Ecutoff to 40Ry, the  
> energies values are actually more accurate but the problem persist.
>
> Hania
>
>
>  &control
>     calculation  = 'scf'
>     restart_mode = 'from_scratch'
>     pseudo_dir   = '/root/pwscf/pseudo/'
>     outdir       = '/root/tmp/'
>  /
>  &system
>     ibrav=7
>     celldm(1)=7.33, celldm(3)=8.54
>     nat=19
>     ntyp=3
>     ecutwfc=25.0
>     ecutrho=100.0
>  /
>  &electrons
>     conv_thr = 1e-6,
>     mixing_beta=0.2,
>     startingwfc=’random’
>  /
> ATOMIC_SPECIES
>   Bi    208.98    Bi.pbe-d-mt.UPF
>   Ti    47.867    Ti.pbe-sp-van_ak.UPF
>   O     15.9994   O.pbe-van_ak.UPF
> ATOMIC_POSITIONS
>  Bi 0.00000000 0.00000000 0.5713404
>  Bi 0.00000000 0.00000000 7.9816596
>  Bi 0.00000000 0.00000000 1.8072489
>  Bi 0.00000000 0.00000000 6.7457511
>  Ti 0.00000000 0.00000000 3.1757289
>  Ti 0.00000000 0.00000000 5.3772711
>  Ti 0.00000000 0.00000000 4.2765000
>  O  0.50000000 0.00000000 0.0000
>  O  0.00000000 0.50000000 0.0000
>  O  0.50000000 0.00000000 2.13825
>  O  0.50000000 0.00000000 6.41475
>  O  0.00000000 0.00000000 3.771873
>  O  0.00000000 0.00000000 4.781127
>  O  0.00000000 0.00000000 2.7241305
>  O  0.00000000 0.00000000 5.8288695
>  O  0.50000000 0.00000000 1.0049775
>  O  0.50000000 0.00000000 7.5480225
>  O  0.00000000 0.50000000 1.0049775
>  O  0.00000000 0.50000000 7.5480225
> K_POINTS {automatic}
>   4 4 4 1 1 1
>
>
>
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://www.sissa.it/~baroni / [+39] 040 3787 406 (tel) -528 (fax) /  
stefanobaroni (skype)

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