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[转载]VASP计算磁性、稀土元素电子驰誉不收敛怎么办?

已有 2498 次阅读 2021-7-15 09:52 |系统分类:科研笔记|文章来源:转载

在用VASP做一些磁性体系、稀土元素及相关化合物的静态计算(电子迭代),经常会遇到电子驰誉步骤难收敛的问题。
 
 下面查找了网上的资料,转发几个相关参数调试方法:

  1LMAXMIX
   Default: LMAXMIX = 2
   An additional flag controls up to which l quantum number the onsite PAW charge densities are passed through the charge density mixer. Higher l-quantum numbers are usually not handled by the mixer. In order to obtain fast convergence to the groundstate, you can try the following setting:
 
   LMAXMIX = 4 for d elements
   LMAXMIX = 6 for f elements
 
  
 这个FLAG对于含d电子和f电子的体系是非常重要的,很大一部分体系的收敛问题可以通过设置合适的LMAXMIX值来解决。

  2ALGO, IALGO, LDIAG
   If the self-consistency loop does not converge within 40 steps, it will probably not converge at all. In this case you should reconsider the tags IALGO, LDIAG, and the mixing-parameters.
  
 这是说明书上的建议。
  
 一般情况下,或使用IALGO=48时遇到收敛问题的话,可以考虑设IALGO384.5以前的版本可设为8),或设置ALGO=Normal or Fast (in VAS P.4.5 and later versions)

  3NELMDL
   NELMDL gives the number of non-selfconsistent steps at the beginning; if one initializes the wave functions randomly the initial wave functions are far from anything reasonable. The resulting charge density is also 'nonsense'. Therefore it makes sense to keep the initial Hamiltonian, which corresponds to the superposition of atomic charge densities, fixed during the first few steps.
  Choosing a 'delay' for starting the charge density update becomes essential in all cases where the SC-convergence is very bad (e.g. surfaces or molecules/clusters chains). Without setting a delay VASP will probably not converge or at least the convergence speed is slowed down.
  NELMDL might be positive or negative. A positive number means that a delay is applied after each ionic movement — in general not a convenient option. A negative value results in a delay only for the start-configuration.

  4mixing-parameters
  
 对于一些难收敛的体系,可以使用“linear mixing”,具体详见VASP说明书中的“Mixing-tags”

   For an initial linear mixing (BMIX ~ 0) an optimal setting for A(AMIX) can be found easily by setting Aopt=Acurrent*Γmean. For the Kerker scheme either A or q0(i.e. AMIX or BMIX) can be optimized, but we recommend to change only BMIX and keep AMIX fixed (you must decrease BMIX if the mean eigenvalue is larger than one, and increase BMIX if the mean eigenvalue is smaller than one).
  
  
 尽管VASP说明书中给出了调节AMIXBMIX的一些较为明确的建议,但是实际去调节的时候,还是挺难的,但原则上说,是可以通过调节这两个Flag来使得收敛问题得以解决的,只是得有耐心。

  5kmesh, SIGMA


  
 收敛问题还跟kmeshSIGMA(当使用ISMEAR不等于-5 和-4)的设置有关。要达到同样的精度,较小的SIGMA则需要较大的kmesh;而且,当SIGMA较小时,若kpoints不够多,也会出现难收敛的情况。

附上一些自己关于磁性的文章,供大家参考:

  1.  Rationally designed high-performance spin-filter based on two-dimensional half-metal Cr2NO2. Matter 2019, 5 (1), 1304-1315.  

  2. Coexistence of piezoelectricity and magnetism in two-dimensional vanadium dichalcogenides. Phys. Chem. Chem. Phys. 2019, 21, 132-136. 

  3. High magnetoresistance in ultra-thin two-dimensional Cr-based MXenes. Nanoscale 2018, 10, 19492-19497 



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