The parameters for relaxations are coarse, and I didn’t think they were good enough to give accurate energies for convex hulls. I had thought that Materials Project convex hull energies were calculated from static runs for this reason, but after some exploring, it seems that they’re actually done with energies from relaxations.
I decided to test the convergence by running a StaticFW for the suspect Cs structure, mp-1184151, but I couldn’t converge it electronically. I noticed that even with the increased k-point density of the StaticFW, the k-point generator still only assigned a gamma-point only calculation for this cell, so I increased the k-point grid to 3 x 3 x 3. This calculation returned an energy (E0 in the OSZICAR) of -24.471127 eV, much higher than the relaxation, which gave -25.966. Therefore it seems that the single k-point is inadequate to sample this cell, even approximately. I also tested running an OptimizeFW on mp-1184151, and it timed out after around 30 ionic steps, suggesting that the single gamma point is also inadequate for relaxing this cell.
Is this due to the previously reported issue (Serious problem: incomplete calculations labeled as final) ? I thought that this had been fixed.
In this particular case it seems that it’s not a difference between default OptimizeFW and default StaticFW; instead, they both fail for the large Cs cell. However, in general I would prefer to make convex hulls using energies from static calculations, and ignore phases which don’t have static calculations yet. For example, I noticed that going by relaxations, the mp-21462 structure of Eu is 19 meV/atom more stable than mp-623532, but according to the more accurate static calculation, it is 2 meV/atom less stable. I should be able to exclude relaxation-only phases by using the filters @dwinston described.
Thanks for the help!
P.S. While we’re talking about polymorphs of Cs, do you know why this relaxation stopped after three steps? https://materialsproject.org/tasks/mp-639727#mp-639727 The energy change between the second and third steps is 28 meV, about an order of magnitude larger than EDIFFG for this system.