[Pw_forum] GGA underestimate the lattice constant of BaTiO3
ruizhi zhang
rui_zhi_zhang at yahoo.com
Sat Jul 19 13:57:17 CEST 2008
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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