[Pw_forum] vc-relax time and physical procedure

[Masoud Nahali]

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 Nahali
SUT
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