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[verstraete@pcpm.ucl.ac.be]

Hello Mike,

a few comments, as I've also done a few surface calculations. 

You should try optcell 9: this keeps the c axis fixed (and the vacuum 
layer between your slabs), while allowing an in-plane relaxation.

Using ionmov 2 (you don't need moldyn, it's less efficient) will relax
your atomic positions, but the problem can be that you want a surface
reconstruction. In this case you need a supercell, and you need to try
breaking the symmetry by hand, and letting abinit find a new minimum
itself (or guided by you if you know what you're looking for). I don't
think the calculations we've done so far are questionable, simply if we
haven't allowed for surface reconstruction they may not give the
experimental result, or the absolute minimum. The advantages of enforcing
symmetry in abinit are incommensurable with the small difficulty of
breaking symmetry by hand to see if things reconstruct.

The C-P algorithm is great, but I don't think it is a solution to your
problems. The convergence difficulties in surface calculations, in my
experience, are self-consistent cycle problems, not geometrical relaxation
ones. In the latter context CP would be nice, but the problem is inherent
in the vacuum. The potential there can fluctuate wildly without
influencing the energy, so the convergence is slow (sometimes impossible).
CP would probably have the same problems. A clean-up of the SCF might be
needed, but a more efficient or adapted preconditioning scheme would be
even better (anyone heard of one?).

To boot, the smaller your gap the smaller your CP time step has to be
(hence it could even be slower than Broyden minimization). Schemes exist
to work with metallic systems (VandeVondele & DeVita), but none of this
has been implemented widely by other groups, so your slabs would have to 
be insulating to begin with... I dread the thought: all of my slabs thus 
far have been metallic.

If anyone else has thoughts on the SC convergence in abinit, I'm 
interested.

Ciao

Matthieu

-- 
===================================================================
Matthieu Verstraete               mailto:verstraete@pcpm.ucl.ac.be 
PCPM, Boltzmann, pl. Croix du Sud, 1            tel: 010/ 47 86 81
B-1348 Louvain-la-Neuve  Belgium                fax: 010/ 47 34 52