[Pw_forum] VC-relax collapsing unit cell

[John Bilgerman]

Hi,

I've been banging my head against this and cannot find what is likely a
silly mistake despite many tests and lots of reading.

I'm trying to optimize the (known) structure of NaFePO4 as a test. I'm
starting from the experimental crystal structure, so the drastic collapse
of the unit cell to < 1/2 suggests an issue.

I know the common problem is inputing the structure wrong, but I've done my
best (and sanity-checked the input/output files with Xcrysden).

I'm new to QE, any help would be appreciated.

Input file:
 &CONTROL
                 calculation = 'vc-relax' ,
                restart_mode = 'from_scratch' ,
                      outdir = './' ,
                      wfcdir = './scratch' ,
                  pseudo_dir = './pseudo' ,
                     disk_io = 'default' ,
                   verbosity = 'high' ,
 /
 &SYSTEM
                       ibrav = 8,
                 space_group = 62 ,
                           A = 9.001 ,
                           B = 6.874 ,
                           C = 5.052 ,
                       cosAB = 0 ,
                       cosAC = 0 ,
                       cosBC = 0 ,
                         nat = 6,
                        ntyp = 4,
                     ecutwfc = 35 ,
                     ecutrho = 140 ,
                 occupations = 'smearing' ,
                     degauss = 0.02 ,
                    smearing = 'gaussian' ,
                       nspin = 2 ,
   starting_magnetization(1) = 0.7,
   starting_magnetization(2) = 0,
   starting_magnetization(3) = 0,
   starting_magnetization(4) = 0,
                    noncolin = .false. ,
 /
 &ELECTRONS
             diagonalization = 'david' ,
 /
 &IONS
 /
 &CELL
 /
ATOMIC_SPECIES
   Fe   55.00000  Fe.pbe-sp-hgh.upf
    P   30.00000  P.pbe-hgh.upf
   Na   22.00000  Na.pbe-sp-hgh.upf
    O   16.00000  O.pbe-hgh.upf
ATOMIC_POSITIONS crystal_sg
Fe 4a
P 4c 0.17585  0.46447
Na 4c  0.34999  0.9702
O 8d 0.1212 0.0682 0.3177
O 8d 0.3486 0.25 0.4561
O 8d 0.1154 0.25 0.7507
K_POINTS automatic
  2 3 4   1 1 1


The relevant parts of the CIF file for the structure are:
...
_cell_length_a 9.001(8)
_cell_length_b 6.874(3)
_cell_length_c 5.052(4)
_cell_angle_alpha 90.
_cell_angle_beta 90.
_cell_angle_gamma 90.
_cell_volume 312.58
_cell_formula_units_Z 4
_symmetry_space_group_name_H-M 'P n m a'
_symmetry_Int_Tables_number 62
...
Fe1 Fe2+ 4 a 0 0 0 . 1. 0
P1 P5+ 4 c 0.17585(4) 0.25 0.46447(8) . 1. 0
Na1 Na1+ 4 c 0.34999(9) 0.25 0.9702(2) . 1. 0
O1 O2- 8 d 0.1212(1) 0.0682(1) 0.3177(2) . 1. 0
O2 O2- 4 c 0.3486(1) 0.25 0.4561(2) . 1. 0
O3 O2- 4 c 0.1154(1) 0.25 0.7507(2) . 1. 0
...

John
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