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« on: October 7, 2016, 11:28 »
Hi Cyrille,
I addition to the comments by Ulrik and Petr:
1) Where are the maximum forces located? Are they on the core of the molecular network, or on the hydrogen atoms? Hydrogen atoms can be quite painful to relax, because their movement leads only to tiny changes on the potential energy surface, but you have a lot of H atoms in your structure.
2) You are using the PBE functional, which - by personal experience - gives rather flat minima for H-bonded potential energy surfaces, or more in general surfaces involving H atoms.
3) you are using a SZP basis set for all atoms, which might also results in a poor description of the potential energy surface, if the minimum in the potential energy surface turns out to be rather flat.
My suggestion is that in the spirit of quantum-chemistry calculations, you perform the optimization stepwise: first a rough (0.05 eV - 0.025 eV/Ang) optimization with SZP basis set, then a more accurate (0.01 eV/Ang) optimization using a SZP or better an SG15 basis set (Medium or High). Notice that SG15 basis sets are more long ranged than FHI basis sets, so they might work better for system with rather flat potential energy surfaces.
Cheers,
Daniele.
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