On 11/03/2015 02:47 PM, Laurence Marks wrote: > a) :FCHECK (bottom of case.scf) was large. This is the sum of all > the forces, and should be small. Particularly for cells without > inversion one can get bad, highly asymmetric densities in which case > MSR1a can have problems.
True, I had not notice this before. :FCHECK was already quite large (hundreds for x and y, even >1000 for z) before ‘clminter’ and “jumped” to even larger values afterwards. > b) The greed is small. Too small a value can be as bad as too large. > I have struggled with this for years and failed to find a strong > ansatz for this, although I believe the next release of the mixer > will be better. AFAIK, the greed in MSR1(a) is set internally by the mixer (the corresponding value in case.inm being ignored), so there is nothing I can do directly to influence this, is there? > I am not sure what the calculation is, perhaps an oxide surface > where you have made a guess at the initial structure and want to > minimize to something more reasonable. Quite a good guess, it is an oxide heterostructure including a slab of vacuum, but the initial structure is derived (cut out) from a converged one from an older calculation, so I would have expected only relatively minor adjustments. As such, the large forces also come as something of a surprise. > I strongly suggest trying to use cells with inversion, they behave > much, much better. In this case, inversion symmetry could only be achieved by adding an additional “film” on the back side of the “substrate”. > I also strongly suggest that you look at the Bond Valence Sums (BVS) > and tweak the initial positions until they are reasonable. (x nn ; > grep Bond *tnn). If, for instance, you have highly underbonded O > (e.g. 0.8) it can take forever and the calculations can be unstable > -- convergence is faster the more physical are the atomic positions. I guess the expectation is that the BVS should be close to the “formal valence” of the ion, right? In this case, it seems okay: The first BVS value is between 1.4 and 2.2 for O, deviations for other species are rather smaller). > Good luck. Thanks, and thank you for your tips. Elias -- Elias Assmann Institute of Theoretical and Computational Physics TU Graz ⟨https://itp.tugraz.at/⟩
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