[Pw_forum] vc-relax

[udayagiri sai babu]

Dear all I wanted to run some vc-relax calculations. To start with I took
pbtio3 system and tried to relax it. for the following input file it
predicted the c/a ratio as 1.23. The output structure (both coordinates and
lattice parameters) is grossly in error. In the forum i have seen people
reporting c/a ratio which is in good agreement with the experimental value.
Later I realized for different input configurations it is predicting
different c/a ratios. At one time for pbtio3 I got cubic structure after
relaxing!!!. As i understood it is stuck up in local minimum. Since i know
the structure is tetragonal i could figure out the error but for an unknown
system how can i relax the system with confidence.
(if there is some literature that i can read up on this please refere me the
same i will read)

entering subroutine stress ...

          total   stress  (Ry/bohr**3)                   (kbar)     P=
0.38
   0.00000259   0.00000000   0.00000000          0.38      0.00      0.00
   0.00000000   0.00000259   0.00000000          0.00      0.38      0.00
   0.00000000   0.00000000   0.00000248          0.00      0.00      0.36


     bfgs converged in  34 scf cycles and  27 bfgs steps

     End of BFGS Geometry Optimization

     Final enthalpy =    -333.0490579145 Ry

CELL_PARAMETERS (alat)
   0.983673643   0.000000000   0.000000000
   0.000000000   0.983673643   0.000000000
   0.000000000   0.000000000   1.208160764

ATOMIC_POSITIONS (crystal)
Ti       0.500000000   0.500000000   0.562608151
O        0.500000000   0.500000000  -0.068487782
O        0.500000000   0.000000000   0.453495153
O        0.000000000   0.500000000   0.453495153
Pb       0.000000000   0.000000000   0.118889324


&control
calculation = 'vc-relax',
restart_mode='from_scratch',
outdir='/home/sai/pbtio3/'
prefix='pbtio3'
pseudo_dir='/home/sai/softwares/espresso-4.0.1/pseudo'
tstress=.true.,
/
&system
ibrav=0
nat=5,
ntyp=3,
ecutwfc=55,
ecutrho=550,
nbnd=22,
occupations='fixed',
degauss=0.00
/
&electrons
conv_thr=1.D-6,
mixing_beta= 0.3
/
&ions
/
&cell
cell_dynamics='bfgs'
/
ATOMIC_SPECIES
Pb  207.21  Pb.pbe-d-van.UPF
Ti 47.87  Ti.pbe-sp-van_ak.UPF
O  16  O.pbe-rrkjus.UPF
ATOMIC_POSITIONS {crystal}
Ti 0.5 0.5 0.52
O 0.5 0.5 0
O 0.5 0 0.5
O 0 0.5 0.5
Pb 0 0 0
K_POINTS {automatic}
8 8 7 0 0 0
CELL_PARAMETERS
7.374 0 0
0 7.374 0
0 0 7.669

-- 
U.Saibabu
PhD student,
Deformation mechanisms modeling group,
Materials engineering department,
IISc Bangalore,
India.
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