[Pw_forum] vc-relax (one dimension supercell)

[Giuseppe Mattioli]

Dear REI

Please sign always your posts to this forum with name and scientific affiliation.

>     forc_conv_thr = 1.d-6
>     etot_conv_thr = 1.d-7,

VERY tight convergence criteria. You could never find convergence

>     celldm(1) = 40.50311907 ,

Huge distance between your wires. If you want to save time perform a first vc-relax calculation with a shorter distance (i.e., 25~30 a.u.), then use 
the optimized positions and z lattice parameter to start a new calculation with a longer xy spacing.

>     ecutwfc =80.0,

Probably too high for Si, but I did not test the psudopotential, so I cannot say for sure. Anyway a too high cutoff does not hurt!

>     diagonalization = 'cg'

Why not davidson? cg should be used only when davidson fails

>     conv_thr =  1.0d-12,

VERY VERY tight convergence criterium. You probably do not need such an accuracy on wfcs. 1.0d-8 (which becomes 1.0d-10 around bfgs convergence) is 
generally more than adequate for vc-relax calculations.

>     bfgs_ndim = 3,

Possibly useless in this case...

>   cell_dofree = 'xyz' ,

The most important thing: you should optimize the z parameter only, which is meaningful in the case of your system. Use  cell_dofree = 'z'

> K_POINTS AUTOMATIC
> 1 1 16 0 0 0

You may start with a poorer mesh (i.e. 1 1 4, or 1 1 8), and find convergence on the k-points mesh step by step. It is not mandatory, but it usually 
saves time...

HTH

Giuseppe

On Tuesday 02 July 2013 05:05:35 REI.BOND wrote:
> Dear all,
> 
> (sorry! Since pre- mail has been garbled, it is sent once again. )
> 
> I have a problem.
> 
> My problem of the 16 atom supercell system ( one-dimension system ), I
> am trying to vc-relax.
> 
> First, when it calculates in a supercell, is it amusing that pressure
> is applied from a vacuum layer? (in my calculation, vacuums layer is x
> and y)
> 
> Then, if  cell_dofree = 'xyz' is used, a vacuum layer will become small
> rapidly.
> 
> Does my calculation have any problems?
> 
> 
> Thanks,
> 
> REI
> 
> ---input----
> 
>  &control
>     calculation='vc-relax'
> !    restart_mode='restart',
>     tprnfor = .true.
>     tstress = .true.
>     prefix='110',
>     pseudo_dir = './',
>     outdir='./'
>     forc_conv_thr = 1.d-6
>     etot_conv_thr = 1.d-7,
>     nstep =100000
>  /
>  &system
>     ibrav=6,
>     celldm(1) = 40.50311907 ,
>     celldm(3) = 0.180919129 ,
> !    celldm(3) = 4.362274784 ,
>     nat=  14,
>     ntyp= 2,
>     ecutwfc =80.0,
>  /
>  &electrons
>     diagonalization = 'cg'
>     electron_maxstep = 100
>     conv_thr =  1.0d-12,
>     mixing_beta = 0.2,
>  /
>  &ions
>     bfgs_ndim = 3,
>  /
>  &cell
>   cell_dofree = 'xyz' ,
>   press_conv_thr=0.1
>  /
> ATOMIC_SPECIES
>    Si 28.086     Si.pz-vbc.UPF
>    H 1.00782503207 H.pz-vbc.UPF
> ATOMIC_POSITIONS bohr
> Si       6.680764370   8.898916109   5.986070989
> Si       9.029846753   5.231537887   5.363083201
> Si       6.680057629   1.564614309   5.986065052
> Si       4.221207325   8.899147367   2.313388654
> Si       1.871413129   5.232223481   2.936367832
> Si       4.220504860   1.564853884   2.313381924
> H        8.432421873  11.126743390   6.027800731
> H        2.468854211  -0.662974560   2.271647941
> H        2.469974729  11.127305994   2.271650709
> H        8.431281864  -0.663551556   6.027799087
> H       -0.711612701   5.232463267   1.803772875
> H        1.465680126   5.232255329   5.753598846
> H       11.612877425   5.231282938   6.495661694
> H        9.435578196   5.231491779   2.545852436
> K_POINTS AUTOMATIC
> 1 1 16 0 0 0
> 
> ----out---
> 
> End of self-consistent calculation (1st)
> 
> 
>      Forces acting on atoms (Ry/au):
> 
>      Total force =     0.003197     Total SCF correction =     0.000003
> 
> 
>      entering subroutine stress ...
> 
>           total   stress  (Ry/bohr**3)                   (kbar)     P=  
> -0.04 -0.00000100   0.00000000  -0.00000056         -0.15      0.00    
> -0.08 0.00000000  -0.00000123   0.00000000          0.00     -0.18     
> 0.00 -0.00000056   0.00000000   0.00000137         -0.08      0.00     
> 0.20
> 
> CELL_PARAMETERS (alat= 40.50311907)
>    0.999975086   0.000000000   0.000000000
>    0.000000000   0.999969153   0.000000000
>    0.000000000   0.000000000   0.180925314
> 
> End of self-consistent calculation (23st)
> 
> Forces acting on atoms (Ry/au): Total force = 0.005428 Total SCF correction
> = 0.000022 entering subroutine stress ... total stress (Ry/bohr**3) (kbar)
> P= 0.39 0.00000356 0.00000000 -0.00000051 0.52 0.00 -0.07 0.00000000
> 0.00000351 0.00000000 0.00 0.52 0.00 -0.00000051 0.00000000 0.00000096
> -0.07 0.00 0.14 CELL_PARAMETERS (alat= 40.50311907) 0.805107566 0.000000000
> 0.000000000 0.000000000 0.809022136 0.000000000 0.000000000 0.000000000
> 0.181542885


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   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
   v. Salaria Km 29,300 - C.P. 10                
   I 00015 - Monterotondo Stazione (RM)          
   Tel + 39 06 90672836 - Fax +39 06 90672316    
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