[Pw_forum] GGA underestimates lattice constant in BaO

[Munima Bora Sahariah]

Dear PWscf users,

For last couple of days I have been trying to find the equilibrium lattice
constant of BaO with GGA (pw91) and has found that  my calculated value
underestimates the experimental one by 1.3% (GGA = 10.301 a.u., EXP =
10.431 a.u.). Surprizingly, while trying with LDA, I obtained a value
which is closer to the experimental one as compared to GGA (LDA = 10.324
a.u.).

I have taken the pseudopotentials from pwscf and vanderbilt's uspp site.
My earlier calculations of lattice constants in BeO with GGA showed very
good agreement with the experimental values.

While doing a search on the web, I found one paper (Phys. Rev. B 71,
085203) where GGA and LDA results for lattice constant of BaO have been
listed. In this paper calculations were done in CASTEP. Here the LDA value
underestimates the experimental lattice constant while the GGA(pw91) value
improves the situation and is closer to the experimental one.

Since my calculations are going against the trend and also against some
published result, I would like to seek the help of the forum. Below I have
attached my scf input file.

with regards

munima

*********************** scf input file *********************************

&control
    calculation = 'scf'
    restart_mode='from_scratch',
    prefix='bao',
    pseudo_dir = '~/pseudo/',
    outdir='./tmp/'
 /
 &system
    ibrav=  2, celldm(1) =$lc, nat=  2, ntyp= 2,
    ecutwfc =35.0, ecutrho =400, occupations ='fixed',
 /
 &electrons
    diagonalization='cg'
    mixing_mode = 'plain'
    mixing_beta = 0.7
    conv_thr =  1.0d-8
 /
ATOMIC_SPECIES
 Ba  137.33 Ba.pw91-nsp-van.UPF
 O 15.9994 O.pw91_n.UPF
ATOMIC_POSITIONS
 Ba 0.00 0.00 0.00
 O 0.50 0.50 0.50
K_POINTS {automatic}
  8 8 8 1 1 1

**********************************************************************


Dr. Munima B. Sahariah
PI, DST Fast Track Project
IASST, Guwahati
Assam, INDIA


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