Here is a vc-relax calculation example.

&control

In this section, we are going to define the calculation type to 'vc-relax' instead of 'scf'.

'etot_conv_thr' is the convergence threshold on total energy (a.u) for ionic minimization. The default values for'etot_conv_thr' is 1.0D-4.

'forc_conv_thr' is the convergence threshold on forces (a.u) for ionic minimization. The default values for'forc_conv_thr' is 1.0D-3. To get better convergence, the example uses smaller values for 'etot_conv_thr' and 'forc_conv_thr'.

'nstep' is the number of molecular-dynamics or structural optimization steps performed in this run. The default value is 50.[1]


&system

'ecutrho' is kinetic energy cutoff (Ry) for charge density and potential. The default value is 4*'ecutwfc'. However, a larger value (8 to 12 times 'ecutwfc') is typically used.


&cell

The input difference between 'relax' and 've-relax' is 'relax' does not have ' &cell' section. In this section, the type of cell dynamics is deifned.

 &control
    calculation='vc-relax',
    restart_mode='from_scratch',
    prefix='Bi',
    outdir='temp',
    pseudo_dir = '.',
    etot_conv_thr=1.0d-6,
    forc_conv_thr=1.0d-6,
    nstep=100,
/
 &system    
    ibrav= 5, celldm(1) =8.979, celldm(4) =0.538, nat= 2, ntyp= 1,
    ecutwfc =60.0, ecutrho =720.0,
    occupations='smearing', smearing='mp', degauss=0.06
/
 &electrons
    mixing_beta =0.7,
    conv_thr =1.0d-8,
/
 &ions
    ion_dynamics='bfgs'
/
 &cell
    cell_dynamics='bfgs',
    press=0.0,
    press_conv_thr=0.5,
/
ATOMIC_SPECIES
 Bi  208.9804 Bi.pbe-dn-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS (crystal)
 Bi 0.2673 0.2673 0.2673 
 Bi 0.7327 0.7327 0.7327
K_POINTS automatic
 19 19 19 0 0 0 

Reference

1. Quantum-espresso.org. (2019). pw.x: input description. [online] Available at: https://www.quantum-espresso.org/Doc/INPUT_PW.html [Accessed 19 Apr. 2019]