[Pw_forum] negative value of polarization and Z* for Pb in Tetragonal PbTiO3 and discrepancy

[Prasenjit Ghosh]

Dear Sathya,

First of all I would like you to note that the method used in the PRL is
different from that used in QE, while the former is based on LAPW method,
Pwscf is a plane wave code. So the structural parameters given in the PRL is
a good guess to start your calculation. However, you should first obtain the
optimized values of cell parameters and atomic positions using pwscf and
then proceed to do other calculations. These values should be comparable to
those mentioned in the PRL, provided you use same level of theory (eg. same
exchange correlation, same pseudopotential, same k-points, etc.) and you do
the things correctly.

As mentioned by others, you should test your pseudopotentials and also
perform convergence tests for k-point mesh used, ecutwfc, etc. Moreover,
since you are using ultrasoft pseudo potential, I would suggest you to use
ecutrho=8-10*ecutwfc, presently u r using ecutrho=4.0*ecutwfc, which is
often not sufficient for US psps. At least  for structural relaxations,
"conv_thr=1e-08" is sufficient, although you may need to use a more strict
condition when you calculate the effective charges.

Hope this helps.

Prasenjit.

PRASENJIT GHOSH,
POST-DOC,
ROOM NO: 265, MAIN BUILDING,
CM SECTION, ICTP,
STRADA COSTERIA 11,
TRIESTE, 34104,
ITALY
PHONE: +39 040 2240 369 (O)
+39 3807528672 (M)

2008/7/11 sathya subramanyan <sathya_sheela1985001 at rediffmail.com>:

>
>
>
> Dear Prof. Eyvaz,
>                   Thank you very much for your reply In response to your
> advice regarding the inaccuracy in structure input for tetragonal PbTiO3, I
> checked the literature and thus have taken  the structural input
> parameteres(lattice constant,c/a and atomic positions)from PRL 80,1998,4321
> and done the convergence test. conv_threshold used now is 1e-12
>   The result is as follows.Could you please let me know whether the
> convergence is good or not.
>
> THE INPUT FILE FOR TETRAGONAL PbTiO3
>
> &control
>     calculation  = 'scf'
>     restart_mode = 'from_scratch'
>     pseudo_dir  = '/home/nbaski/espresso-4.0.1/pseudo/'
>     outdir      = '/home/nbaski/tmp/'
> /
> &system
>     ibrav=6
>     celldm(1)=7.373,
>     celldm(3)=1.065
>     nat=5
>     ntyp=3
>     nbnd=25
>     ecutwfc=30.0
>     occupations = 'fixed'
>     degauss=0.00
> /
> &electrons
>     conv_thr = 1e-12,
>     mixing_beta=0.3,
> /
> ATOMIC_SPECIES
>   Pb  207.2      Pb.vdb.UPF
>   Ti    47.867    Ti.vdb.UPF
>   O    15.9994  O.vdb.UPF
> ATOMIC_POSITIONS
>   Pb    0.000    0.000    0.000
>   Ti    0.500    0.500    0.538
>   O    0.500    0.000    0.612
>   O    0.000    0.500    0.612
>   O    0.500    0.500    0.117
> K_POINTS {automatic}
>   4 4 4 1 1 1
>
>                     **************
> THE OUTPUT FILE(a part only as it is a heavy file)
>
> highest occupied, lowest unoccupied level (ev):    9.6746  12.5554
>
> !    total energy              =  -333.71409381 Ry
>     Harris-Foulkes estimate  =  -333.71409381 Ry
>     estimated scf accuracy    <        5.8E-13 Ry
>
>     The total energy is the sum of the following terms:
>
>     one-electron contribution =  -94.69940998 Ry
>     hartree contribution      =    73.25826560 Ry
>     xc contribution          =  -49.63383021 Ry
>     ewald contribution        =  -262.63911922 Ry
>
>     convergence has been achieved in  19 iterations
>
>
> SATHYA SHEELA.S
> Grad. Student
> Department of Physics
> National Institute of Technology
> Tiruchirapalli - 620015
> India
>
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