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/ The early bird gets the worm, but the second mouse gets the cheese. - S. Wright If you go through a lot of hammers each month, I don't think it necessarily means you're a hard worker. It may just mean that you have a lot to learn about proper hammer maintenance - J. Handey
