[Pw_forum] vc-relax time

[Masoud Nahali]

  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
>
 Sincerely Yours
 Masoud Nahali
 SUT
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