[QE-users] Questions on Applying Pressure in VC-MD Simulations

[Nam Tran]

Dear QE Experts,

I am trying to create an amouphous structure (Na2SiO3) using a melting-cooling process. I use "vc-md" for my calculation but the obtained density seems to be incorrect.
In more detail, I start with orthogonal cell containing 138 atoms to maintain the density of 2.6 g/cm^3 [1]. In the melting process, I run the vc-md (NPT) at 5000 K and pressure of 1 atm (0.00 1kbar). After several MD steps the cell shrinks at lot causing the density increases to 4.77 g/cm^3.
Therefore, I wonder I have done something wrong with the input. Maybe, I have misunderstood that "press = 0.001" is something else but not the external pressure acting on the cell.

I appreciate any help and advice

Regards

........................................................................................

INPUT

&CONTROL
 calculation='vc-md',
 restart_mode = 'from_scratch',
 tstress = .true.,
 nstep =20000,
 dt = 20.0,
 outdir='./tmp/',
 prefix='NSO',
 pseudo_dir='=/pseudo/'
/

&SYSTEM
 ibrav = 0,
 nat = 138,
 ntyp = 3,
 ecutwfc=40.0,
 ecutrho=160.0,
 occupations = 'smearing',
 smearing = 'gauss',
 degauss = 0.01,
/

&ELECTRONS
 conv_thr = 1.d-6,
 electron_maxstep = 500,
 mixing_beta = 0.7D0,
 mixing_mode='local-TF',
/

&IONS
 ion_dynamics='beeman',
 ion_temperature = 'andersen',
 tempw = 5000,
/

&CELL
 cell_dynamics='pr',
 press = 0.001,
 cell_dofree = 'z',
/

ATOMIC_SPECIES
 O 15.999 O.pbe-hgh.UPF
Na 22.990 Na.pbe-sp-hgh.UPF
Si 28.085 Si.pbe-hgh.UPF

CELL_PARAMETERS angstrom
17.4419994354     0.0000000000      0.0000000000
0.0000000000      10.0699996948     0.0000000000
0.0000000000      0.0000000000      10.1600000000

K_POINTS gamma

ATOMIC_POSITIONS angstrom
O 14.268993 1.361002 3.297000
O 8.769019 1.382002 3.281000
O 3.069007 1.401002 3.266000
O 14.253031 1.376002 6.496000
....................................


OUTPUT

  ......................................................................................................


     Computing stress (Cartesian axis) and pressure

          total   stress  (Ry/bohr**3)                   (kbar)     P=-5249.70
  -0.05197737  -0.00014172   0.00026844      -7646.13    -20.85     39.49
  -0.00014172  -0.04974923   0.00007203        -20.85  -7318.36     10.60
   0.00026844   0.00007203  -0.00533369         39.49     10.60   -784.61


     Entering Dynamics;  it =    22   time =  0.02033 pico-seconds

  new lattice vectors (alat unit) :
   1.000000000   0.000000000   0.000000000
   0.000000000   0.577342049   0.000000000
   0.000000000   0.000000000   0.319288923
  new unit-cell volume =   6600.8893 (a.u.)^3
  new positions in cryst coord
................................................................
................................................

     Ekin =   122.83179595 Ry    T = ****** K  Etot = -6172.23708296
     new unit-cell volume =   6600.88934 a.u.^3 (   978.15087 Ang^3 )
     density =      4.76597 g/cm^3

[1]. http://www.ilo.org/dyn/icsc/showcard.display?p_version=2&p_card_id=0359
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