[Pw_forum] Finite electric field calculation

[Nicola Marzari]

Dear Hanghui,


Paolo Umari might be better suited to answer the first question - he
has fixed a few things lately, mostly to allow fields in arbitrary
directions (I think).

On the second one, I might be more helpful - as the electric field
increases, or the k-point sampling becomes finer, the system will fall
into a "runaway" solution, and not converge. This is the computational
counterpart of the fact that in a periodic system in an electric field
the energy is strictly unbound from below (that's where you "run away" -
an inescapable fall into the abyss that silently awaits). The ground
states of  the electric enthalpy are interpreted as the metastable
solutions corresponding to a physical system in which you adiabatically
turn on a field, and whose actual lifetimes can be much longer that
any experiment would be.

Details in abundance in the 2002 (?) PRLs of Umari/Pasquarello and
of Souza/Iniguez/Vanderbilt, plus I believe later PRBs from the same
groups.

				nicola


alan chen wrote:
> Dear PWSCF users,
>      I have two questions about finite electric field calculation based 
> on Berry phase.
>      First, I have found in the source code that the formula to calcuate 
> the polarization in the presence of finite electric field is different 
> in Version 4.0 from in Version 3.2. A couple of factors like sqrt(2) and 
> alat are added in the new version. I just want to make sure that if I 
> use ultrasoft psedupotential and turn on finite electric field, does the 
> latest version (4.0) of PWSCF calculate the polarization and atomic 
> force correctly? No offense intended.
>      Second, is there a range of electric field in which the Berry phase 
> method works? In the example31 the electric field is 0.001 a.u. I want 
> to calculate some properties under the electric field as large as 0.01 
> a.u. In principle I think the method works for any value of electric 
> field but in practice any restriction on the maximum of electric field?
>      Thank you very much.
> 
> Hanghui Chen
> Department of Physics
> Yale University
> 
> 
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Prof Nicola Marzari   Department of Materials Science and Engineering
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