[Pw_forum] large error in RuO2 lattice parameter

[mohnish pandey]

Dear QE users,
2.7438
I want to study RuO2 surfaces. I did calculation of ruthenium metal and
found the lattice constant right(experimental = 2.70 and 1.584, I got 2.7438
and 1.582) But after doing calculation of bulk ruthenium oxide I am getting
very different lattice parameter as compared to the experimental value.
Experimental value is 4.494 and 3.107 and I am getting the value of 4.6421
and 3.1888. Can anybody please tell from where the discrepancy is coming?

Thanks a lot in advance

*input file*

&control
    calculation = 'vc-relax',
    restart_mode='restart',
    nstep=100
    outdir='/home/mohnish/Desktop/Ruo2-bulk',
    pseudo_dir='/home/mohnish/Desktop/Ruo2-bulk',
    prefix='Ruo2',
    tstress = .true.,
    tprnfor = .true.,
  /
  &system
    ibrav=  6, a = 4.6379, c = 3.1844,nat=  6, ntyp= 2,
    ecutwfc =
40,ecutrho=400,occupations='smearing',degauss=0.01,smearing='gaussian'
  /
  &electrons
   diagonalization='david'
   mixing_mode = 'plain'
   mixing_beta = 0.7
   conv_thr = 1.0d-8
   startingpot = 'file'
   startingwfc = 'file'
 /
&IONS
 ion_dynamics='bfgs'
/
&CELL
 cell_dynamics = 'bfgs'
/
ATOMIC_SPECIES
  Ru 101.07 Ru.pbe-n-van.UPF
  O  15.099 O.pbe-rrkjus.UPF
 ATOMIC_POSITIONS (crystal)
Ru  0.00000000  0.00000000  0.00000000
Ru  0.50000000  0.50000000  0.50000000
O   0.30600000  0.30600000  0.00000000
O   0.69400000  0.69400000  0.00000000
O   0.19400000  0.80600000  0.50000000
O   0.80600000  0.19400000  0.50000000
 K_POINTS (automatic)
 10 10 12 0 0 0


*Section of output file,*

bfgs converged in   8 scf cycles and   4 bfgs steps
     (criteria: energy < 0.10E-03, force < 0.10E-02, cell < 0.50E+00)

     End of BFGS Geometry Optimization

     Final enthalpy =    -227.8256404205 Ry
Begin final coordinates
     new unit-cell volume =    463.72196 a.u.^3 (    68.71650 Ang^3 )

CELL_PARAMETERS (alat=  8.76436083)
   1.000910330   0.000000000   0.000000000
   0.000000000   1.000910330   0.000000000
   0.000000000   0.000000000   0.687553615

ATOMIC_POSITIONS (crystal)
Ru       0.000000000   0.000000000   0.000000000
Ru       0.500000000   0.500000000   0.500000000
O        0.306258404   0.306258404   0.000000000
O        0.693741596   0.693741596   0.000000000
O        0.193741596   0.806258404   0.500000000
O        0.806258404   0.193741596   0.500000000
End final coordinates

-- 
Regards,
MOHNISH,
-----------------------------------------------------------------
Mohnish Pandey
Senior Project Associate,
Department of Chemical Engineering,
IIT KANPUR, UP, INDIA
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