[Pw_forum] vc-relax in the presence of finite electric field

[alan chen]

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
electric field?
      Thank you very much.

Input file reads:
&CONTROL
 calculation='vc-relax'
 wf_collect=.true.
 pseudo_dir = './psp'
 outdir='/scratch'
 wfcdir='/scratch'
 prefix='sLAO-vc'
 tprnfor = .true.
 tstress = .true.
 disk_io='low'
 verbosity='high'
 dt=80.D0
 lelfield=.true.
 gdir=3
 nppstr=16
 nberrycyc=1
/
&SYSTEM
 ibrav=  6
 celldm(1) = 7.27
 celldm(3) = 0.9506
 nat=  5
 ntyp= 3
 ecutwfc = 30.0
 ecutrho = 180.0
 occupations='fixed'
 nosym=.true.
/&ELECTRONS
 diagonalization='david'
 mixing_beta = 0.7D0
 diago_david_ndim = 4
 startingwfc='random'
 startingpot='atomic'
 efield=0.00348
 conv_thr = 1.0d-9
/
&IONS
 ion_dynamics = 'damp'
 phase_space = 'full'
 pot_extrapolation = 'second_order'
 wfc_extrapolation = 'second_order'
/
&CELL
 cell_dynamics = 'damp-w'
 cell_dofree = 'z'
/
ATOMIC_SPECIES
 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
ATOMIC_POSITIONS crystal
 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
K_POINTS crystal
256
(The k points are omitted here)

Hanghui Chen
Department of Physics
Yale University
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