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[PGanesh <pganesh@ciw.edu>]

Dear D. R. Hamann,

Thank you for the reply.  I have already go the DFPT results for the 
piezo. and elastic constants.  But the values are much lower compared to 
the expected values looking at the energy differences of the diffrent 
stable structures.  So I was thinking if maybe P vs E is non-linear.  Of 
course the piezo's are very sensitive to the volume.

The strange thing is that even if  run the example file in the tutorial 
of non-linear optics (tffield_7.in) I get the same problem.   I wonder 
why that is.  

Are you aware of any paper that does a finite E field calulation to 
study the polarisation rotation?   Does the example file work for you?

Thank you very much.

regards,
PGanesh




D. R. Hamann wrote:
> Dear P. Ganesh,
>
> It is extremely difficult to compute dielectric and piezoelectric 
> constants by finite-difference methods, especially when atomic 
> relaxations are involved (ie., in anything but the simplest unit 
> cells).  I suggest you use the density functional perturbation theory 
> capabilities of Abinit instead, reading and working through the 
> "Response Function 1," "Response Function 2," and "Elastic" tutorials.
>
> Don Hamann
>
> P. Ganesh wrote:
> > Dear All,
> >
> > I am trying to compute polarization vs electric-field.  If I start 
> > with the ground state structure and apply a very small electric-field 
> > (~0.00001 a.u) and relax the ions and cell shape only (i.e optcell = 
> > 3, ionmmov = 2) then the zeroth broyden step converges with a 
> > polarisation along [x0z] direction.  But right after the 1st SCF 
> > cycle in broyden step 1 I get the electronic, ionic and total 
> > polarisation to be exactly zero.  Further, at SCF 7 the polarization 
> > agains become non-zero, and very close to the previous non-zero value 
> > and the program aborts with the following warning message:
> >
> > scfcv (electric field calculation) : WARNING -
> >   The difference between pel (electronic Berry phase updated
> >   at each SCF cycle)
> >   and pel_cg (electronic Berryphase computed using the berryphase 
> > routine) is
> >   pdif_mod =  0.178600608E+01
> >
> > pel_cg(1) =  0.000000000E+00
> > pel_cg(2) =  0.000000000E+00
> > pel_cg(3) =  0.000000000E+00
> > pel(1) = -0.886982093E+00
> > pel(2) =  0.000000000E+00
> > pel(3) = -0.999634356E+00
> >
> > I dont understand why the polarization suddenly goes to zero at SCF 1 
> > and how it suddenly came back to be non-zero? I am using v5.4.4.
> >
> > Thank you very much.
> >
> > PGanesh