[QE-users] Interpreting vc-relax output

[Bharat Thapa]

Dear Developer,

I was running vc-relax of Hf3N4. The input parameters are for body centered
cubic crystal: space group I-43d (group 220 of the international table of
crystallography). I used this space group for the positions of Hf and N
atoms. The main thing I want to ask is: after vc-relax the CELL_PARAMETERS
values which are for ibrav= -3 changed such that all diagonal elements are
same but different to all non-diagonal elements ,however, the non-diagonal
elements are same to each other. I think, if they all were same then that
would mean the unit cell is either shrink or expanded which would make it
easy to calculated the final (relaxed) lattice constant. But, in this case
where it has different numbers for diagonal and non-diagonal, how do I
interpret it? Is it still a cubic lattice? or it is trigonal now? I think I
can not change the atomic positions otherwise I would lose the symmetry.
What do I do now? Is it true that vc-relax may change the atomic positions
but it will not change the crystal symmetry or the space group?

Input:





























































*&control    calculation = 'vc-relax',    restart_mode = 'from_scratch',
forc_conv_thr = 1.0D-5,    tstress = .true.,    tprnfor = .true.,    prefix
= 'Hf3N4_vr3' ,    pseudo_dir =
'/home/z5126106/QEspresso_bin/Pseudopotentials',    outdir =
'/share/scratch/z5126106/TempFiles',    verbosity = 'high'    !disk_io =
"none"  /  &system    ibrav = 0,     celldm(1) = 12.69, !materialsproject
  nat = 14,    ntyp = 2,    ecutwfc = 60,    ecutrho = 720     nbnd = 80
occupations = 'fixed'  /  &electrons    conv_thr = 1.0d-11,    mixing_mode
= 'plain',    mixing_ndim = 10,    mixing_beta = 0.3  /  &ions
ion_dynamics =  "bfgs",    bfgs_ndim    = 3  /  &cell    cell_dynamics =
"bfgs",    press_conv_thr = 0.1D0,    cell_dofree = "all"    cell_factor =
5  /CELL_PARAMETERS alat ! ibrav = -3 -0.5000 0.5000 0.5000 0.5000 -0.5000
0.5000 0.5000 0.5000 -0.5000ATOMIC_SPECIES Hf 178.49
Hf.pbe-spn-kjpaw_psl.1.0.0.UPF N 14.0067 N.pbe-n-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS alat !crystal Hf       0.375000000   0.000000000
0.250000000 0 0 0   Hf       0.125000000   0.000000000   0.750000000 0 0
0Hf       0.250000000   0.375000000   0.000000000 0 0 0 Hf
0.750000000   0.125000000   0.000000000 0 0 0Hf       0.000000000
0.250000000   0.375000000 0 0 0Hf       0.000000000   0.750000000
0.125000000 0 0 0N        0.099800000   0.099800000   0.099800000 N
 0.400200000   -0.09980000   0.599800000 N        -0.09980000   0.599800000
  0.400200000 N        0.599800000   0.400200000  -0.099800000 N
 0.349800000   0.349800000   0.349800000 N        0.150200000   0.650200000
  0.849800000 N        0.849800000   0.150200000   0.650200000 N
 0.650200000   0.849800000   0.150200000 K_POINTS automatic 6 6 6 1 1 1*

Output:































*Begin final coordinates     new unit-cell volume =   1718.69533 a.u.^3 (
254.68437 Ang^3 )CELL_PARAMETERS (alat= 12.69000000)  -0.426131582
0.607025249   0.607025249   0.607025249  -0.426131582   0.607025249
 0.607025249   0.607025249  -0.426131582ATOMIC_POSITIONS (crystal)Hf
0.375000000  -0.000000000   0.250000000    0   0   0Hf       0.125000000
0.000000000   0.750000000    0   0   0Hf       0.250000000   0.375000000
0.000000000    0   0   0Hf       0.750000000   0.125000000  -0.000000000
 0   0   0Hf      -0.000000000   0.250000000   0.375000000    0   0   0Hf
    0.000000000   0.750000000   0.125000000    0   0   0N
 0.048570739   0.048570739   0.048570739N        0.146441100  -0.104982446
  0.427330131N       -0.104982446   0.427330131   0.146441100N
 0.427330131   0.146441100  -0.104982446N        0.309957583   0.309957583
  0.309957583N        0.262463403   0.707381916   1.020329856N
 1.020329856   0.262463403   0.707381916N        0.707381916   1.020329856
  0.262463403End final coordinates     A final scf calculation at the
relaxed structure.     The G-vectors are recalculated for the final unit
cell     Results may differ from those at the preceding step. *
Regards,


*Bharat Thapa*
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