Hello all,

I am trying to calculate the elastic constants of cementite (Fe3C) with Quantum Espresso using inputs generated by ElaStic. However, when I call ElaStic, I have as output:

Number and name of space group: 2 (P -1)
Triclinic structure in the Laue classification.
This structure has 21 independent second-order elastic constants.

But cementite is orthorhombic with 9 independent elastic constants. sgroup is not able to classifiy it correctly because I entered the conventional unit cell (below)? Thanks in advance.

ATOMIC_POSITIONS crystal
C -0.119491674 0.249982623 0.441808604
C 0.119491674 -0.249982623 -0.441808604
C -0.380404252 -0.249973646 -0.057798324
C 0.380404252 0.249973646 0.057798324
Fe 0.041910120 0.250056375 -0.159219104
Fe -0.041910120 -0.250056375 0.159219104
Fe 0.457866115 -0.250061254 0.340943503
Fe -0.457866115 0.250061254 -0.340943503
Fe 0.183569742 0.068664582 0.331995453
Fe -0.183569742 -0.068664582 -0.331995453
Fe -0.316534124 0.431316546 0.168056629
Fe 0.316534124 0.568682454 -0.168056629
Fe -0.183592774 0.568754219 -0.332024228
Fe 0.183592774 0.431244781 0.332024228
Fe 0.316559314 -0.068592706 -0.168095176
Fe -0.316559314 0.068592706 0.168095176

Best regards,

Roberto

Hello

I got the same result but I used atomic positions {crystal_sg}
I agree with Roberto this structure is orthorhombic with 9 independent elastic constants. It even goes so far as to tell me that the ibrav i am using orthorhombic is incompatible. I am attaching my input script. But I really don't know why it is telling me the system is triclinic should i specify something else in the flags so that ElaStic_Setup_ESPRESSO will recognize it as orthorhombic.

&CONTROL
calculation = 'scf'
restart_mode='from_scratch',
prefix='FeCcb',
tstress = .true.
tprnfor = .true.
outdir = './tmp3/' ,
pseudo_dir ='/home/graevelab/Programs/qe-6.4.1/pseudosC/',
/
&SYSTEM
ibrav = 8,
celldm(1) = 9.81493e+00,
celldm(2) = 1.29788e+00,
celldm(3) = 1.14730e+00,
nat = 3,
ntyp = 2,
ecutwfc =40.0,
occupations='smearing',
smearing='marzari-vanderbilt',
degauss=0.05, ecutrho = 320.0,
nspin =2,
starting_magnetization(1) =0.1
space_group = 62,

/
&ELECTRONS
diagonalization='cg'
mixing_mode = 'plain'
mixing_beta = 0.7
conv_thr = 1.0d-6

/
&IONS
ion_dynamics = 'bfgs'

/
&CELL
cell_dynamics = 'bfgs',
press = 0.0,
press_conv_thr = 0.5,

/
ATOMIC_SPECIES
Fe 55.845 Fe.pz-spn-kjpaw_psl.0.2.1.UPF
C 12.0107 C.pz-n-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS {crystal_sg}
Fe 0.18340 0.06890 0.33440
Fe 0.03880 0.25000 0.84220
C 0.87640 0.25000 0.44260

K_POINTS automatic
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