[Pw_forum] vc-relax time

Masoud Nahali masoudnahali at live.com
Tue Jul 13 15:41:00 CEST 2010


 Dear Gabriele and pwscf users


Perfectly I am sure that the positions of atoms is exactly correct. Note
that the input is belong to a (1*1) slab
and indeed you could not see a honeycomb ! but It is easy to see a honeycomb
when you have a (2*2) slab like this:

C        0.000352513   0.001055549   0.000000000
C        2.459305478   0.000933657  -0.008798962
C       -1.229191611   2.130502759  -0.008802160
C        1.229819481   2.130516670  -0.008801637
C        0.000327447   1.420695799  -0.007291549
C       -1.229173562   3.550193436  -0.007292113
C        2.459296954   1.420621766  -0.009715082
C        1.229852979   3.550208684  -0.007292392
C        0.000507119   0.000488868   3.187515573
C        2.459486429   0.000491165   3.187352189
C       -1.228985445   2.130026298   3.187352283
C        1.229997454   2.130026366   3.187352273
C        1.229970852   0.710263879   3.187439366
C        0.000476982   2.839800509   3.187332530
C        3.688943914   0.710263968   3.187439418
C        2.459456202   2.839797012   3.187439363
C        0.000212609   0.000545761   6.403453823
C        2.459191836   0.000545600   6.403451859
C       -1.229278051   2.130084159   6.403451942
C        1.229700861   2.130084205   6.403451921
C        0.000121431   1.420208410   6.403892505
C       -1.229369239   3.549746424   6.403892711
C        2.459100653   1.420208253   6.403888354
C        1.229609673   3.549746469   6.403892772

  a         = 4.9178,
  b         = 4.9178,
  c         = 16.4112,  10 angstrom vacuum


Also,  see the "As.vcs00.in" example in the "VCSexample" folder of quantum
espresso 4.2. In this example a vc-relax calculation
has been performed using the max_seconds and dt keyboard.

Many Thanks

I hope one helps these problems mentioned below.





>Are you completely sure? You can find as an attachment a picture (from
>xcrysden) of the central graphene layer according to the positions given
>in your input. You can now judge if it looks like a honeycomb lattice.

> Yes, indeed the high ecutrho is important for ultrasoft
> pseudopotentials. About the rippling : It was my mistake in selecting
> a wrong pseudopotential which has a hole. Nicola had explained it
> before and accordingly I solved it. The graphite surface is not
> ripple. When I use the "max_second=6000 and dt=150" the job completes
> very fast as the example of  pwscf. Is the using of such keyboards
> plausible?

>Please spend some time reading Doc/INPUT_PW to understand the meaning of
>the _keywords_. dt is used only for molecular dynamics runs, not
>relaxations, while max_seconds has nothing to do with how fast your job
>is completed.


> > Dear  Gabriele Sclauzero and pwscf Users
>
> > Many thanks for your attentions. The cell dimension and the positions of
> >the atoms are exactly correct. Yes, indeed the high ecutrho is important
> for
> >ultrasoft pseudopotentials. About the rippling : It was my mistake in
> >selecting a wrong pseudopotential which has a hole. Nicola had explained
> it
> >before and accordingly I solved it. The graphite surface is not ripple.
> When
> >I use the "max_second=6000 and dt=150" the job completes very fast as the
> >example of  pwscf. Is the using of such keyboards plausible?
>   >As I mentioned before I had used the optimized cell parameters of (1*1
> >slab) for vc-relaxing the (2*2 slab) and I expected to see the results
> very
> >soon but the calculation was time consuming while there was only a very
> very
> >bit change of the cell dimensions during this 16 hours. There is only 1-3
> >iterations per each step in the output file and each of them was time
> >consuming. The job was completed after about 50 steps.
>
> -------------------------------------------------------------------------
>
> > Dear Masoud,
> >
> >   first I would suggest you to use bfgs as the algorithm for both ions
> and
> > cell dynamics. Excepted particular cases, it should reach the minimum
> much
> > faster.
> >
> > Also, why do you specify the cell with such an unusual way. You simply
> need
> > celldm(1) and celldm(3) with ibrav=4 if you want to describe an hexagonal
> > lattice. Other suggestions: your ecutrho looks really large to me, do you
> > really need it. On the other hand, degauss might be too large to describe
> a
> > spin-polarized system.
> >
> > Then, are you sure that you have built correctly your supercell? It looks
> > like there are some C-C bonds much shorter that others in the central
> > graphene plane (1.2 instead of 1.4 angs). Please check again your
> structure.
> > In general, you can expect that if you relax the atoms in the supercell
> > some kind of surface-reconstruction may appear, since you leave more
> freedom
> > to atoms to rearrange in structures with larger periodicity. I don't
> think
> > this is the case for graphite, but you may find some ripples (as you
> > mentioned in your earlier emails, if I am not wrong) if the C-C bonds are
> at
> > a distance shorter than the theoretical equilibrium distance (I guess).
> >
> > HTH
> >
>
> ------------------------------------------------------------------------------------------------
>  Dear Quantum Espresso Users
> >
> > I vc-relaxed a (1*1) slab of graphite surface with 3 layers; It takes 20
> minutes with parallel running by 4 CPUs. Then I used the exact optimized
> cell parameters (obtained from vc-relaxed calculation) to make a (2*2) slab
> of graphite with 3 layers and I expected to see the results in a few
> minutes. But amazingly it took 17 hours to complete. 48 steps were done in
> the calculation for vc-relaxing the cell which have the parameters that had
> been optimized before. The cell parameters only change a very bit in the
> current vc-relaxing the (2*2) slab. I appreciate if one explain the
> physical
> procedure of vc-relaxing and the reason of the time needed for the
> computation.
> >
> > input file:
> >
> > CONTROL
> >   calculation  = "vc-relax",
> >   pseudo_dir   = "/home/koa/soft/qe4.2/
> > espresso-4.2/pseudo",
> >   outdir       = "/home/koa/tmp",
> >   etot_conv_thr= 1.0D-4,
> >   forc_conv_thr= 1.0D-3,
> >   dt=80,
> >     /
> > &SYSTEM
> >   ibrav     = 4,
> >   a         = 2.4579,
> >   b         = 2.4579,
> >   c         = 16.3069,
> >   cosab     = -0.5,
> >   cosac     = 1.0,
> >   cosbc     = 1.0,
> >   nat       = 6,
> >   ntyp      = 1,
> >   ecutwfc   = 40.D0,
> >   ecutrho   = 480.D0,
> >   occupations = 'smearing'
> >   smearing ='mp',
> >   degauss = 0.03,
> >   nspin = 2,
> >   starting_magnetization(1)= 0.003,
> >   london=.true.,
> >   /
> > &ELECTRONS
> >   conv_thr    = 1.D-6,
> >   mixing_beta = 0.7D0,
> >   diagonalization = "david",
> > /
> > &IONS
> >  ion_dynamics="cg"
> > /
> > &CELL
> > cell_dynamics = 'damp-w',
> > press = 0.0,
> > /
> > ATOMIC_SPECIES
> > C  12.0107  C.pbe-rrkjus.UPF
> > ATOMIC_POSITIONS {angstrom}
> >  C                  0.00000000    0.00000000    0.00000000  1 1 0
> >  C                  0.00000000    1.41908472    0.00000000
> >  C                  0.00000000    0.00000000    3.15347111
> >  C                  11.22896342    0.70954236    3.15347111
> >  C                  0.00000000    0.00000000    6.30694222
> >  C                  0.00000000    1.41908472    6.30694222
> > K_POINTS {automatic}
> > 4 4 1 1 1 1
> >
>
>
>
>
>
>
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