How to do geometrical optimization of Silicon

 

There are two types of structural optimization calculations in Quantum espresso: (1) relax: where only the atomic positions are allowed to vary, and (2) vc-relax: which allows to vary both the atomic positions and lattice constants.

We will do the total geometrical optimization. We need 2 files

algerien1970@linux-wipm:~/abinitio/QE-tutorials/Si.vc-relax> ls
Si.pz-vbc.UPF  si.vc-relax.in

si.vc-relax.in

&control
    calculation = 'vc-relax'
    prefix = 'silicon'
    outdir = 'Si_OUT/'
    pseudo_dir = './'
    etot_conv_thr = 1e-5
    forc_conv_thr = 1e-4
/

&system
    ibrav=2, celldm(1) =14,
    nat=2, ntyp=1,
    ecutwfc=30
/

&electrons
    conv_thr=1e-8
/

&ions
/

&cell
    cell_dofree='ibrav'
/

ATOMIC_SPECIES
  Si  28.0855  Si.pz-vbc.UPF

ATOMIC_POSITIONS (alat)
  Si 0.00 0.00 0.00 0 0 0
  Si 0.25 0.25 0.25 0 0 0

K_POINTS (automatic)
  6 6 6 1 1 1

Si.pz-vbc.UPF

You can download the file using the following command

$ wget https://www.quantum-espresso.org/upf_files/Si.pz-vbc.UPF --no-check-certificate

 

Doing geometrical optimization

algerien1970@linux-wipm:~/abinitio/QE-tutorials/Si.vc-relax> pw.x < si.vc-relax.in |tee si.vc-relax.out 

This produces following output (see the si.vc_relax.out for more details, look for "Final enthalpy"):

Final enthalpy           =     -15.8536258729 Ry

     File Si_OUT/silicon.bfgs deleted, as requested
Begin final coordinates
     new unit-cell volume =    265.83803 a.u.^3 (    39.39313 Ang^3 )
     density =      2.36778 g/cm^3

CELL_PARAMETERS (alat= 14.00000000)
  -0.364530890   0.000000000   0.364530890
   0.000000000   0.364530890   0.364530890
  -0.364530890   0.364530890   0.000000000

ATOMIC_POSITIONS (alat)
Si            0.0000000000        0.0000000000        0.0000000000    0   0   0
Si            0.1822654448        0.1822654448        0.1822654448    0   0   0
End final coordinates

Lattice constant = 0.364530890 * 14 / 0.5 = 10.20686492 Bohr.

 

 

 Reference: Using PWscf: basics (write-up, exercises, March 2018)