[Pw_forum] Dipole correction in vacuum, polar slabs

[Conor Hogan]

Dear developers/users,

I have a few questions and observations on the dipole correction to the
spurious macroscopic field that arises in polar slabs+vacuum calculations,
as implemented in PWSCF. As far as I can see, the only place this has been
discussed before has been in this forum:
http://www.democritos.it/pipermail/pw_forum/2004-July/001135.html , in
some papers (e.g. Journal of Chemical Physics (2004) 120, p9934), and in
the source code PW/add_efield.f90 PW/compute_dip.f90.

1) Reference to this facility seem to be completely removed from the
documentation (distinct, of course, from the facility to add an external
electric field, tefield =.true.). In particular, the fact that the flag
dipfield = .true. is needed is only mentioned in the source (took me a
while to find that out!). Is this a conscious decision? Is the dipole
correction now "unsupported"? Was it ever "supported"? Is the routine
known to be buggy? One of the main reasons I started using PWSCF is for
this reason...

2) I see some weird behaviour when the dipole field is used, and I'd like
to know if it's a bug, or if it arises from a real physical mechanism.
What I've observed is that convergence depends on the relative position of
the slab within the supercell: if the slab is roughly symmetric about
zero, and the dipole layer located at about 0.5 of the cell, the
correction works, and convergence is achieved; if the slab and layer are
shifted, the convergence is never reached and the total energy shoots to
large positive values.
At first appearance, this to me is a bug. However, it also occurred to me
that it might just be related to the old problematic definition of the
dipole moment within the infinite lattice. Has anyone any idea why this
behaviour arises? If the potential converges, can I trust the calculation?

Thanks, and best regards to all.
Conor


----

Dr. Conor Hogan
Dipartimento di Fisica e CNR-INFM
Universita' di Roma "Tor Vergata"
Tel: +39 06 72594548
Fax: +39 06 2023507
http://www.fisica.uniroma2.it/~cmtheo-group/

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