[QE-users] Problem with vc-relax simulation of Pb (cell dimensions are much smaller than they should be)

[sarashs]

Hi quantum espresso users,

I am performing a vc-relax simulation on pb. I have included my input 
file for your convenience. The problem that I have is that the minimum 
energy cell is more than 1 Angstrom smaller on each dimension (3.25 
Angstrom) while it should be around 4.95. I am wondering if you can help 
me figure out why? I have tried increasing the number of K points, 
ecutwfc and I tried with different degauss values but I was not 
successful in converging to the right size.

Is this an xc functional issue? or something else?

&control
   calculation =   "vc-relax"   ,
    restart_mode = 'from_scratch' ,
    outdir='$TMP_DIR/' ,
    pseudo_dir = '$PSEUDO_DIR' ,
    disk_io = 'default' ,
    verbosity = 'default' ,
    tstress = .true. ,
    tprnfor = .true. ,


  /
  &system
     ibrav = 2, celldm(1) =4.9505, nat= 1, ntyp= 1,
     ecutwfc = 25.0, ecutrho = 300.0
     occupations='smearing', smearing='gaussian', degauss=0.02
  /
  &electrons
     diagonalization='david'
     conv_thr = 1.0e-8
     mixing_beta = 0.7
  /
&IONS
  /
  &CELL
    cell_dynamics = 'bfgs',
    press = 1.00 ,
  /
ATOMIC_SPECIES
  Pb 207.2 Pb.pbe-dn-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS alat
  Pb 0.0 0.0 0.0
K_POINTS (automatic)
  5 5 5 0 0 0


Best regards,

S. Arash Sheikholeslam