[Pw_forum] vc-relax in the presence of finite electric field
chenhanghuipwscf at gmail.com
Wed Aug 13 20:46:15 CEST 2008
Dear PWSCF users,
I have done a calculation of LaAlO3 recently, using 'vc-relax' with
the contraints that only z-direction is relaxed and x,y directions are
fixed, and in the presence of finite electric field (based on Berry Phase
method). The input file is attached at the end.
However I am very confused with the results. The equilibrium lattice
constant in z direction is c=0.95*a (a is the lattice constant in x and y
direction). The final ionic polarization is 72.03 a.u. and electronic
polarization is 4.50 a.u. Then I use the equilibrium unit cell (c=0.95*a)
and just 'relax' the atoms in the presence of the same electric field. I
obtain: ionic polarization is 75.13 a.u. and electronic polarization is 1.36
a.u. Even more confusing is that if I use the equilibrium unit cell
(c=0.95*a) and put the atom in the ideal position (i.e. no ionic
polarization), the reference ionic polarization is 73.24 a.u.
Shall I trust the 'vc-relax' calculation in the presence of finite
Thank you very much.
Input file reads:
pseudo_dir = './psp'
tprnfor = .true.
tstress = .true.
celldm(1) = 7.27
celldm(3) = 0.9506
ecutwfc = 30.0
ecutrho = 180.0
mixing_beta = 0.7D0
diago_david_ndim = 4
conv_thr = 1.0d-9
ion_dynamics = 'damp'
phase_space = 'full'
pot_extrapolation = 'second_order'
wfc_extrapolation = 'second_order'
cell_dynamics = 'damp-w'
cell_dofree = 'z'
La 138.91 057-La-ca-nsp-hanghui.uspp.format.UPF
Al 26.98 013-Al-ca--hanghui.uspp.format.UPF
O 16.00 008-O-ca--vgrp.uspp.format.UPF
La 0.00 0.00 0.00
Al 0.50 0.50 0.50
O 0.50 0.50 0.00
O 0.00 0.50 0.50
O 0.50 0.00 0.50
(The k points are omitted here)
Department of Physics
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