Born effective charge and dielectric constant calculation in finite electric field:

References :
<li>Responses in the presence of an applied electric field
<br><cite> XJ Wang and D. Vanderbilt, Phys. Rev. B 75, 115116 (2007).</cite>
<br><cite> XJ Wang and D. Vanderbilt, Phys. Rev. B 74, 054304 (2006).</cite>

The calculations require three steps:

1. run a scf ground-state calculation at zero electric field
   to get wave functions for the ground-state calculation in finite electric
   fields.

2. run a scf ground-state calculation in finite electric field. The
   electric field is controlled by the input variable 'efield'. And
   'berryopt' should be 4. The input variable 'kptopt' should be set
   to be 2.

3. Based on the ground-state wave functions obtained in step (2), linear response
   calculations of phonon perturbation and electric field perturbation are performed.
   The critical input variables are:
  rfphon30   1
 rfatpol30   1 2
   rfdir30   1 1 1
  rfelfd30   3      #turn on electric field perturbation in finite electric field.
    nqpt30   1
  kptopt30   3
berryopt30   4
    nsym30   1      #use no symmetry
     qpt30   0.0 0.0 0.0

   The choice of k-point sampling N x N x N should be the same in the three steps
   and N is restricted to the even integers (2,4,6,8...).