[Pw_forum] dipole correction-dipole field keep constant during optimization

[Guido Fratesi]

Dear Fenggong,

 > Yes, it is a slab. The problem is it is difficult to converge if the
 > dipole correction is added because in our system the charge density is
 > changing too abruptly. Therefore, I was wondering whether adding a
 > dipole correction which is keep constant during the scf convergence can
 > improve the convergence problem. Of course, this will depends very much
 > on the initial value of the dipole field.


that happened to me at once when the sawtooth potential was mistakenly 
added at the wrong place, for example with the ramp inside the system 
rather than in vacuum.

If this is not the case, "difficult" cases can be converged stepwise. E.g.:
  1) do a scf calculation without dipfield (this converges, does it?)
  2) set startingpot='file' and do a scf calculation with dipfield
I did this several times to accelerate convergence for alkali atoms on 
metals. It also worked for structural relaxations (several scf steps), 
because once you had a good charge density the calculations went on 
smoothly.

Or, as you suggest (I never tried)
  1b) do a scf calculation with some guessed electric field ("external", 
variable eamp, no need to edit the code I guess)
  2) as above, now with eamp=0.

Please let us know if that solves your problem.
Guido

-- 
Guido Fratesi

Dipartimento di Scienza dei Materiali
Universita` degli Studi di Milano-Bicocca
via Cozzi 53, 20125 Milano, Italy