[QE-users] Convergence problem

[Akash Saxena]

Dear all users,

Warm Greetings!

I have been calculating the formation energy for the double-cation 
perovskite system, FA(1-x)Cs(x)PbI3. I have taken x to be 0.125, 0.25, 
0.375, 0.5, 0.625, 0.75, 0.875, and 1.

Out of these compositions, x=0.75 variable-cell relaxation calculation 
is not converging. I am sharing my input file below:-

  &CONTROL
   calculation = 'vc-relax'
   etot_conv_thr =   1.0000000000d-06
   forc_conv_thr =   1.0000000000d-03
   prefix = 'facspbi3'
   tprnfor = .true.
   tstress = .true.
   nstep = 200
   verbosity = 'high'
   pseudo_dir = '/home/pmlab/akashsaxena/fapi/facspbi3/pseudopot'
   outdir = '/home/pmlab/akashsaxena/fapi/facspbi3/0.75/lda/outdir_1'
/
&SYSTEM
   degauss =   2.0000000000d-02
   ecutwfc =   6.0000000000d+01
   ibrav = 0
   nat = 54
   nosym = .false.
   ntyp = 6
   occupations = 'fixed'
   smearing = 'gaussian'
/
&ELECTRONS
   conv_thr =   1.0000000000d-06
   electron_maxstep = 200
   diagonalization = 'cg'
/
&IONS
   ion_dynamics = 'bfgs'
/
&CELL
   cell_dynamics = 'bfgs'
/
ATOMIC_SPECIES
Cs  132.905   Cs_pbe_v1.uspp.F.UPF
C   12.0107 C.pbe-n-kjpaw_psl.1.0.0.UPF
H   1.00794 H.pbe-rrkjus_psl.1.0.0.UPF
I   126.90447 I.pbe-n-kjpaw_psl.0.2.UPF
N   14.0067 N.pbe-n-radius_5.UPF
Pb  207.2 Pb.pbe-dn-kjpaw_psl.0.2.2.UPF
ATOMIC_POSITIONS crystal
H            0.7500000000       0.3734180000       0.7500000000
H            0.7500000000       0.8734180000       0.2500000000
H            0.9020890000       0.2876760000       0.7500000000
H            0.9020890000       0.7876760000       0.2500000000
H            0.8486280000       0.1624660000       0.7500000000
H            0.8486280000       0.6624660000       0.2500000000
H            0.6513720000       0.1624660000       0.7500000000
H            0.6513720000       0.6624660000       0.2500000000
H            0.5979110000       0.2876760000       0.7500000000
H            0.5979110000       0.7876760000       0.2500000000
Pb           0.0000000000       0.4927880000       0.0000000000
Pb           0.0000000000       0.4927880000       0.5000000000
Pb           0.0000000000       0.9927880000       0.0000000000
Pb           0.0000000000       0.9927880000       0.5000000000
Pb           0.5000000000       0.4927880000       0.0000000000
Pb           0.5000000000       0.4927880000       0.5000000000
Pb           0.5000000000       0.9927880000       0.0000000000
Pb           0.5000000000       0.9927880000       0.5000000000
C            0.7500000000       0.2881400000       0.7500000000
C            0.7500000000       0.7881400000       0.2500000000
I            0.2500000000       0.4986690000       0.0000000000
I            0.2500000000       0.4986690000       0.5000000000
I            0.2500000000       0.9986690000       0.0000000000
I            0.2500000000       0.9986690000       0.5000000000
I            0.7500000000       0.4986690000       0.0000000000
I            0.7500000000       0.4986690000       0.5000000000
I            0.7500000000       0.9986690000       0.0000000000
I            0.7500000000       0.9986690000       0.5000000000
I            0.0000000000       0.2420610000       0.0000000000
I            0.0000000000       0.2420610000       0.5000000000
I            0.0000000000       0.7420610000       0.0000000000
I            0.0000000000       0.7420610000       0.5000000000
I            0.5000000000       0.2420610000       0.0000000000
I            0.5000000000       0.2420610000       0.5000000000
I            0.5000000000       0.7420610000       0.0000000000
I            0.5000000000       0.7420610000       0.5000000000
I            0.0000000000       0.4848850000       0.2500000000
I            0.0000000000       0.4848850000       0.7500000000
I            0.0000000000       0.9848850000       0.2500000000
I            0.0000000000       0.9848850000       0.7500000000
I            0.5000000000       0.4848850000       0.2500000000
I            0.5000000000       0.4848850000       0.7500000000
I            0.5000000000       0.9848850000       0.2500000000
I            0.5000000000       0.9848850000       0.7500000000
N            0.8388620000       0.2415790000       0.7500000000
N            0.8388620000       0.7415790000       0.2500000000
N            0.6611370000       0.2415790000       0.7500000000
N            0.6611370000       0.7415790000       0.2500000000
Cs           0.2500010000       0.7845050000       0.7500000000
Cs           0.7500000000       0.2881400000       0.2500000000
Cs           0.2500000000       0.2881400000       0.2500000000
Cs           0.7500000000       0.7881400000       0.7500000000
Cs           0.2500000000       0.2881400000       0.7500000000
Cs           0.2500000000       0.7881400000       0.2500000000
K_POINTS automatic
3 3 3 0 0 0
CELL_PARAMETERS angstrom
      13.1578080000       0.0000000000       0.0000000000
       0.0000000000      12.7973780000       0.0000000000
       0.0000000000       0.0000000000      13.1185190000

In the output file, a large value for " negative_rho " is repeatedly 
reported, following which the program reports that the eigenvalues have 
not converged. I have attached a screenshot of the end of the output 
file to this email.
Could anyone suggest where I am going wrong in the input file?

Regards

Akash, Ph.D. Scholar, IIT Bombay.
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