# [Pw_forum] Finite electric field calculation

Nicola Marzari marzari at MIT.EDU
Mon Aug 4 07:34:23 CEST 2008

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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
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu

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