[Pw_forum] forces in the 'sawtooth' method

[Stefano Baroni]

Dear Chen: just two remarks that may or may not be relevant.
1) The "measured" forces seem to me rather small in magnitude. There  
may be accuracy issues. I would try to increase the value of the  
electric field and check that estimated effective charges are  
numerically stable in the range of the linear regime (this check  
should always be done when estimating derivatives numerically).
2) I am out of town at this time, without any textbook handy, so I  
cannot check the correct definitions and current nomenclature.  
However, in the literature one defines two different "effective  
charges" which differ by a factor equal to the dielectric constant  
(basically, one is the force generated by an unit *applied* field, the  
other is the one generated by a unit *internal* field. You may want to  
check the consistency of what you calculate with what the phonon code  
provides.
Cheers - SB

On Oct 4, 2008, at 2:10 AM, alan chen wrote:

> Dear PWSCF users,
>    In order to check whether the forces in the 'sawtooth' method are  
> calculated correctly in the latest version-4.0.2(no offense  
> intended), we carry out the following test. We calculate an isolated  
> symmetric 5-layer STO slab (input file is attached below). In the  
> absence of electric field, there are no forces on the middle TiO2  
> layer due to symmetry. Then we turn on a weak electric field (0.0001  
> a.u.) and obtain the forces on Ti and on O are 0.00046 and -0.00013  
> Ry/au, respectively (in the middle TiO2 layer). Therefore the Born  
> effective charges of Ti and O are approximately
>
> e*(Ti)=0.00046/0.0001/2= 2.3
> e*(O)=-0.00013/0.0001/2= -0.65
>
> The factor '2' is due to the units. However from both phonon  
> frequency calculation and literature, the Born effective charges of  
> Ti and O are:
>
> e*(Ti)= 7.25
> e*(O)= -2.06
>
> I am just wondering whether my method is correct. If it is correct,  
> does that imply there might be something wrong with the forces in  
> the 'sawtooth' method.
>
> Thank you very much.
>
> Hanghui Chen
> Department of Physics, Yale University
>
> &CONTROL
>   calculation='scf'
>   wf_collect=.true.
>   pseudo_dir = './psp'
>   outdir='.'
>   wfcdir='/scratch'
>   prefix='STO-5.5uc-EF0.0001-scf'
>   tprnfor = .true.
>   tstress = .true.
>   disk_io='low'
>   verbosity='high'
>   dt=80.D0
>   tefield=.true.
> /
> &SYSTEM
>   ibrav=  6
>   celldm(1) = 7.27
>   celldm(3) = 10.0
>   nat=  27
>   ntyp= 3
>   ecutwfc = 30.0
>   ecutrho = 180.0
>   occupations='smearing'
>   smearing=gauss
>   degauss=0.005
>   edir=3
>   emaxpos=0.8
>   eopreg=0.1
>   eamp=0.0001
> /
> &ELECTRONS
>   diagonalization='david'
>   mixing_beta = 0.7D0
>   diago_david_ndim = 4
>   startingwfc='atomic'
>   startingpot='file'
>   conv_thr = 1.0d-8
> /
> &IONS
>   ion_dynamics = 'damp'
>   phase_space = 'full'
>   pot_extrapolation = 'second_order'
>   wfc_extrapolation = 'second_order'
> /
> &CELL
>   cell_dynamics = 'damp-w'
> /
> ATOMIC_SPECIES
>  Sr  87.62   038-Sr-ca-sp-vgrp.uspp.format.UPF
>  Ti  47.90   022-Ti-ca-sp-vgrp.uspp.format.UPF
>  O   16.00   008-O-ca--vgrp.uspp.format.UPF
> ATOMIC_POSITIONS alat
>  Sr 0.00 0.00 0.00
>  O  0.50 0.50 0.00
>  Ti 0.50 0.50 0.50
>  O  0.00 0.50 0.50
>  O  0.50 0.00 0.50
>  Sr 0.00 0.00 1.00
>  O  0.50 0.50 1.00
>  Ti 0.50 0.50 1.50
>  O  0.00 0.50 1.50
>  O  0.50 0.00 1.50
>  Sr 0.00 0.00 2.00
>  O  0.50 0.50 2.00
>  Ti 0.50 0.50 2.50
>  O  0.00 0.50 2.50
>  O  0.50 0.00 2.50
>  Sr 0.00 0.00 3.00
>  O  0.50 0.50 3.00
>  Ti 0.50 0.50 3.50
>  O  0.00 0.50 3.50
>  O  0.50 0.00 3.50
>  Sr 0.00 0.00 4.00
>  O  0.50 0.50 4.00
>  Ti 0.50 0.50 4.50
>  O  0.00 0.50 4.50
>  O  0.50 0.00 4.50
>  Sr 0.00 0.00 5.00
>  O  0.50 0.50 5.00
> K_POINTS {automatic}
> 8 8 1 0 0 0
>
>
>
>
>
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://www.sissa.it/~baroni / [+39] 040 3787 406 (tel) -528 (fax) /  
stefanobaroni (skype)

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