[Pw_forum] vc-relax time

[Gabriele Sclauzero]

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

GS


Il giorno 12/lug/2010, alle ore 20.24, Masoud Nahali ha scritto:

> 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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§ Gabriele Sclauzero, EPFL SB ITP CSEA
   PH H2 462, Station 3, CH-1015 Lausanne

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