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[verstraete@pcpm.ucl.ac.be]

diemac 10000 is probably useless: either leave the default 10^6 or (my 
recommendation) set to a small value (1 to 10). Even if the slab is 
metallic in plane, this will sometimes improve convergence, since it's an 
insulator out-of-plane. Otherwise try iprcel 45.

It looks like you only have ca. 5 angstroems of vacuum. That's probably 
not enough, and is in the "bad" transition region where the interaction is 
not negligible but not strong enough to converge quickly. 7 A is usually 
safe, 10 is better, but you should be converging on a sensitive quantity: 
forces or the potential in the middle of the vacuum. A number of mails in 
the archive have already dealt with slabs and vacuum and the related 
problems.

Matthieu

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Matthieu Verstraete             mailto:verstraete@pcpm.ucl.ac.be
PCPM, Boltzmann, pl. Croix du Sud, 1        tel: +32 10 47 33 59
B-1348 Louvain-la-Neuve  Belgium            fax: +32 10 47 34 52