下面以Si晶体为例，进行晶格常数优化。

Input file:

&control
    calculation = 'vc-relax'
    prefix = 'si'
    outdir = './outdir'
    pseudo_dir = '/BIGDATA1/ac_iphy_jrsun_1/soft/QE/SSSP'
    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-7
/
&ions
/
&cell
    cell_dofree='ibrav'
/
ATOMIC_SPECIES
 Si  28.0855  Si.pbe-n-rrkjus_psl.1.0.0.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)
  4 4 4 0 0 0

New Notes:
‘vc-relax’: Cell changed relax step.
‘outdir’: folder name for storing output
‘etot_conv_thr’: energy difference judge between two successive ionic steps
‘forc_conv_thr’: force difference judge between two successive ionic steps
‘cell_dofree=’ibrav’’: all axis and angles are moved, but the lattice remains consistent with the initial ibrav choice

Output: si.vc_relax.out

Begin final coordinates
     new unit-cell volume =    282.09879 a.u.^3 (    41.80273 Ang^3 )
     density =      2.23129 g/cm^3

CELL_PARAMETERS (alat= 14.00000000)
  -0.371816826   0.000000000   0.371816826
   0.000000000   0.371816826   0.371816826
  -0.371816826   0.371816826   0.000000000

ATOMIC_POSITIONS (alat)
Si            0.0000000000        0.0000000000        0.0000000000    0   0   0
Si            0.1859084130        0.1859084130        0.1859084130    0   0   0
End final coordinates

Then we obtain the optimized lattice constant of 14.00000000*0.371816826*2 = 10.4109 Bohr
and the density is 2.23129 $g/cm^3$.