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MAXMIX = [integer]
Default: MAXMIX = -45
MAXMIX specifies the maximum number of vectors stored in the Broyden/Pulay mixer, in other words it corresponds to the maximal rank of the approximation of the charge dielectric function build up by the mixer. MAXMIX can be either negative or positive:
- The mixer is reset after each ionic step or if the number of electronic steps exceeds |MAXMIX| (this is the default and similar to the behavior of VASP.4.3 and VASP.3.2).
- The charge density mixer is only reset if the storage capabilities are exceeded. The reset is done "smoothly" by removing the five oldest vectors from the iteration history. Therefore, if MAXMIX is positive, the approximation for the charge dielectric function which was obtained in previous ionic steps is "reused" in the current ionic step, and this in turn can reduce the number of electronic steps during relaxations and MD's. Especially for relaxations which start from a good ionic starting guess and for systems with a strong charge sloshing behavior the speedup can be significant. We found that for a 12 Å long box containing 16 Fe atoms the number of electronic iterations decreased from 8 to 2-3 when MAXMIX was set to 40. For a carbon surface the number of iterations decreased from 7 to 3. At the same time the energy stability increased significantly. But be careful, this option increases the memory requirements for the mixer considerably, and thus the option is not recommended for systems were charge sloshing is negligible anyway (like bulk simple metals). The optimal setting for MAXMIX is usually around three times the number of electronic steps required in the first iteration. Too large values for MAXMIX might cause the code to crash (because linear dependencies between input vectors might develop).
Mind: MAXMIX is only available in VASP.4.4 and newer versions, and it is strongly recommended to use this option for molecular dynamics and relaxations.