[Pw_forum] Question about vc-relax

["Alejandro Rébola"]

Hello everybody,

I'm trying to do a structural optimization for FeSe (tetragonal). I want
pwscf to determine the optimal a and c and also one internal parameter:
the z coordinate for Se. I'm trying to do a vc-relax calculation, but I
get an error, apparently because I'm not including a Cell card. When I ran
over the examples for vc-relax, a Cell card is included. However, in this
case pwscf also changes the angles, and what I want is to keep the cell as
tetragonal and only look for the optimal a, c and z. Does anybody know
what I should do?
Thank you in advance for your help.
Best,

Alejandro

&control
    calculation= "vc-relax",
    pseudo_dir ='/home/alejandro/quantum_espresso/espresso-4.1.3/pseudo1',
outdir='/scratch/alejandro/espresso'
    verbosity = 'high',
    prefix='FeSe',
 /
 &system
    ibrav= 6,
    a=3.7,
    c=5.5,
    nat= 4, ntyp= 2,
    ecutwfc = 60.0, ecutrho = 350.0,
    occupations='smearing', smearing='mp', degauss=0.01,
 /
 &electrons
    mixing_beta = 0.5
 /
 &IONS
   ion_dynamics      = "damp",
   pot_extrapolation = "second_order",
   wfc_extrapolation = "second_order",
 /
ATOMIC_SPECIES
 Fe  1.  Fe.pbe-nd-rrkjus.UPF
 Se  1.  Se.pbe-van.UPF
ATOMIC_POSITIONS crystal
 Fe 0.75  0.25  0.0  0  0  0
 Fe 0.25  0.75  0.0  0  0  0
 Se 0.25  0.25  0.2  0  0  1
 Se 0.75  0.75 -0.2  0  0  1
K_POINTS automatic
 8 8 6 0 0 0