[Pw_forum] Biaxial Strian Silicon

[Cameron Foss]

Hello all,

I am trying to study phonons in silicon under biaxial strain, which occurs
when you grow thin films of silicon over a germanium(or SiGe) bulk, which
has a slightly larger lattice constant than silicon, thus stretching the
silicon out. This type of procedure stretches silicon in the xy directions
and slightly compresses it in the z direction. This consequently distorts
the symmetry of the crystal.

I have ran some strained cases; in which I "strain" the crystal by changing
the lattice vectors in the CELL_PARAMETERS CARD and the atomic positions
like so:
ATOMIC_POSITIONS {alat}
Si      0       0       0
Si      0.255   0.255   0.25
CELL_PARAMETERS {alat}
  -0.51     0.00    0.5
   0.00     0.51    0.5
  -0.51     0.51    0.00

Whilst the lattice constant remained the equilibrium lattice constant
5.431Angs.

However, I would like to use the vc-relax calculation to relax the
structure to the strained amount...so I know that the structure is
optimized. Although I keep getting an orthogonal operation error.

Here is the vc-relax input code and the error:
&control
    calculation='vc-relax'
    restart_mode='from_scratch',
    outdir='/home/cameron/QE/espresso-5.1/wktmp',
    pseudo_dir='/home/cameron/QE/espresso-5.1/pseudo',
    prefix='relax'
/
&system
    ibrav=2, celldm(1)=10.263, nat=2, ntyp=1,
    ecutwfc =24.0
/
&electrons
    conv_thr =  1.0d-10
    mixing_beta = 0.4
/
&ions
    ion_dynamics='damp'
/
&cell
    cell_dynamics='damp-w',
    press=55.0,
    cell_dofree='2Dxy'
/
ATOMIC_SPECIES
 Si  28.0855 Si.pz-vbc.UPF
ATOMIC_POSITIONS
 Si 0.00 0.00 0.00
 Si 0.25 0.25 0.25
K_POINTS AUTOMATIC
4 4 4 1 1 1

 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
     Error in routine checkallsym (2):
     not orthogonal operation
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

I have two main questions: Did I incorrectly manually "strain" the lattice
structure in the first case?
And how can I use/fix the vc-relax code to simulate the strained structure?

Best,
Cameron
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