[Pw_forum] GGA underestimate the lattice constant of BaTiO3

[ruizhi zhang]

Dear all
 
I calculated the lattice constant of BaTiO3 using GGA. However, the calculated lattice constant ( 3.980 angstrom) is SMALLER than experimental results (4.012 angstrom). So is this result reasonable?
 
The GGA results of SrTiO3 is ok, 3.928 angstrom vs expermental result  3.905. As SrTiO3 and BaTiO3 both contain Ti and O, I calculated lattice constant of bulk metal Ba as a test, and the GGA results (5.014 angstrom for bcc cell) is also smaller than experimental one (5.019).
 
So is there something wrong with my calculation? Or GGA does underestimate the lattice constant of some metals, such as reported in Computer Coupling of Phase Diagrams and Thermochemistry 28(2004)79-90 ?
 
The input file is attached below. Thank you very much for your help 
Ruizhi 

===========input file of BaTiO3===============
&control
    calculation  = 'vc-relax'
    restart_mode = 'from_scratch'
    prefix = 'bto'
    pseudo_dir   = '~/pseudo/'
    outdir       = './tmp/'
 /
 &system
    ibrav=1
    celldm(1)=7.584
    nat=5
    ntyp=3
    ecutwfc=30.0
    occupations = 'smearing'
    degauss=0.01
 /
 &electrons
    conv_thr = 1e-8
    mixing_beta=0.3
 /
&IONS
  ion_dynamics = 'damp',
 /
 &CELL
  cell_dynamics ='damp-pr'
 
 /
ATOMIC_SPECIES
  Ba    137.33  Ba.pbe-nsp-van.UPF 
  Ti    47.90   Ti.pbe-sp-van_ak.UPF
  O     15.99   O.pbe-van_ak.UPF
ATOMIC_POSITIONS (crystal)
  Ba    0.000    0.000    0.000
  Ti    0.500    0.500    0.500
  O     0.000    0.500    0.500
  O     0.500    0.500    0.000
  O     0.500    0.000    0.500
K_POINTS {automatic}
  8 8 8 1 1 1
=============end input file=============

========================
Ruizhi Zhang, PhD
School of Physics,Shandong University
Jinan, Peopel's Republic of China


      
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