于一些磁性体系、镧系和锕系元素及相关化合物的静态计算（电子迭代），经常会遇到“难收敛”的问题。
  下面给出几个相关Flag及设置方法：（有些话从说明书上摘录的，不大好翻译，就copy了，请见谅）

  1、LMAXMIX
   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。

  2、ALGO, 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IALGO为38（4.5以前的版本可设为8），或设置ALGO=Normal or Fast (in VASP.4.5 and later versions)。

  3、NELMDL
   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.

  4、mixing-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AMIX和BMIX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。

  5、kmesh, SIGMA

   收敛问题还跟kmesh及SIGMA(当使用ISMEAR不等于-5 和-4时)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。

   P.S. 对于不同系统的计算，问题的原因不一定一样，因而可能解决之一问题的方法也不一定会一样。其实以上大部分都来自VASP的说明书，只是我在遇到收敛问题时试过这些方法.

   写得比较仓促，以后再修改及补充。
   希望有这方面经验的朋友也把自己的见解提出来一起讨论。

金属表面体系。换算法，改Mixing都不管用。 
 
收敛和k-mesh有关系吗？谢谢了

别改Mixing。 
你把IBRION设为3，然后把SMASS设为0.6试试看

不好意思，我没说清楚 
IBRION是对应离子步的算法 
我的问题是电子步不收敛 
dE 一直0.1左右反复, 200步也不收敛 
改IALGO为38, 增加NBANDS都不行:(

那你试试不要采用自恰计算。但是仍然采用damping驰豫。 
是不是你的初始构型离平衡态太远了。

还没遇到过这种问题，可能比较运气吧。 
调一下cutoff试试。 
IALGO我一直用48，5x5x1 Kpoints。我做金属的表面体系都是在100步内就OK了。 
如果愿意，可以把INCAR贴出来，让大家出出注意，这样更可能有的放矢。

应该不会 
 
有的时候甚至弛豫出来的结构再进行static的计算都不收敛

收敛跟k-mesh是有一定关系的。 
 
你INCAR中没有设置ISMEAR，应该就是使用默认的ISMEAR=1。此时SIGMA应该也是默认值0.2。 
你看看OUTCAR中的EENTRO的值，看是不是符合要求（一般要求小于1 meV/atom，这与你想要的精度有关，如果你要求精度不高的话，这个标准可以设大一些）。 
 
有几个方向你可以试试，或许会有用： 
1、如果EENTRO的值够小，你可以尝试加大SIGMA。（SIGMA较大时，相对收敛会快些） 
2、增加k点多。 
 
其实就是说，要调整k-mesh和SIGMA。一般SIGMA跟k点数会有一定的关系。 
若想达到同一精度，SIGMA越小所需要的K点数就越多。如果SIGMA较小，而k点数不足，有可能会出现难收敛的情况，也可能得到不准确的结果。 
 
 
3、调节AMIX与BMIX 
我之前也遇到过难收敛的问题，当时调这两个参数，还是有点用的。具体调节的方法VASP上有讲到一些，但可能还是要多试才知道。我现在一时想不起来细节了，你先试试，如果有情况，再一起讨论。 
 
VASP的说明书也有提到，如果电子迭代超过40次仍未收敛的话，一般就很难收敛了。当然，我想这只是VASP说明书的一个建议，并不是绝对的。一般设置NELM=60（默认）就够了，如果60次都不能收敛，那可能就要找找问题或是改相应的设置了。 
 
 
有不对或不妥的地方，大家帮忙指正：）

从你的INCAR来看，貌似没什么问题。如果还是不收敛的话，你可以增加一点截断能，比如400 eV，并把POTIM减小至0.1。另外，添加三个tag:NELM=100； NELMDL=6； WEIMIN=0.

大家遇到收敛问题怎么解决啊？？？
小弟仔细读过使用手册，并结合一些帖子，可是还是解决不了问题。恳请高手指点。
以下是我所用的方法和遇到的问题：
解决方法一：设置scf参数
a1.   Iterations. 加大循环次数。计算过程中默认50次，我的体系无论加多大次数就是不收敛。
    可能这不是主要原因。我现在一般设置一个中等大小的值400。
a2.   mixing.对强阻尼体系，降低mixing值到0.01-0.05.甚至用mixing=0.001,diis n=0.保存TAPE21文件，
    再用mixing=0.03,diis n=10.我都用了，都不收敛。分析结果计算过程中有能量跳跃，因此我分析我的体系就
    是较强的阻尼，因此我设置mixing=0.03，这个因该可以吧？
a3.   lshift, vshift.手册上未解释怎么用。 这个我不懂。请问在什么情况下用？怎么用？大家一般设置值为多少？
解决方法二：用occupations指定电子占据。
b1.   直接指定分子占据，由于小弟的量化基础薄弱，弄不懂怎么指定。
   我是按论坛上的方法之一，先算一个单点能，然后得到所有不可约表示的电子占据数。但是，
  我计算单点能虽然scf收敛了，但是计算得到的能量明显不对，估计是收敛到激发态上去了。请问怎么指定到基态？
 另外，对电子占据大家都有什么具体的着？哪位有这方面的文献和资料？我算单点的输出文件在附件中，请高手帮我
 指定一下基态的电子占据。先谢谢了！！！
    这是最重要，最有用的方法，可是我不懂，郁闷啊！！！
b2.   用keeporbitals或freeze控制能量跳跃，但是我的体系用这个关键词还是无法得到几何结构收敛。我只用过
    keeporbitals=20,10,5. 但是scf有几个收敛，可是最后得不到几何结构收敛。
    大家一般设置值为多少？freeze又该如何设置？
b3.   smearq. 手册上说先用smearq=一个值。几何收敛后再用smearq=0, restart 前面的TAPE21文件。
    我设置 smearq=0.2,几何结构收敛了，但是我用smearq=0,restart不收敛。我用smearq=0.1计算连几何结构
    都不收敛。请问大家设置smearq=多少合适？用smearq且令smearq=0之后的结果可信度有多大啊？我看过几篇文献，
    但是都没有什么有用的东西。

我已收到你的邮件了,个人觉得在这么多方法中用smearq 再 occupations比较有效:

先用启用smearing (例如:smearq=0.02), 再根据结果文件指认电子占据一般都能解决问题, 例如:
occupations
A1 a//b
B1 a//b
E  a//b
.
.
.
end.

Dear Vasp Community:

I'm trying to do some very basic defect calculations in a class of complex oxides with the stoichiometry AB6O12, where A=U,W,Re and B=Y,Yb,Lu. In most cases, especially when A=W, I haven't had any problems. But, I have had problems with convergence in two cases that I haven't been able to make any progress on.

When A=U and B=Y, I can converge the perfect structure very nicely. But, when I look at an antisite defect, by replacing one Y with U, the calculation doesn't converge. The electronic iterations get stuck, changing by +/- the same amount. A number of colleagues have suggested I change the smearing, set ISPIN=2, set LMIXMAX=6, etc. Everything I've tried, either doesn't change the behavior or does fix the electronic iterations, but after 3 or 4 ionic iterations, the forces on the ions blow up (become unphyiscally large, like 100 eV/angstrom).

The second problem, when A=Re and B=Lu, is that I don't get any kind of convergence, even in the perfect case. I get a warning at the top

VERY BAD NEWS! internal error in subroutine IBZKPT: 
Reciprocal lattice and k-lattice belong to different class of lattices. 168

But all I've done compared to the A=U, B=Y case is change the POTCAR file. If I let it go, I get similar convergence problems as before.

Any suggestions on what I might try to fix these problems would be greatly appreciated. I'm using ISYM=0 and a Monkhorst-Pack grid of 2x2x2 for these calculations, along with the GGA PAW pseudopotentials.

Thank you, 
Blas  
Back to top    
 
 
admin   Tue Jan 08 2008, 01:27PM    
 

posts 1815
 1) for the electronic convergence 
--) LMAXMIX=6 should be set if you have rare-earth elements in the cell, for d-elements, please set it to 4 
--) it may help to decrease the mixing parameters (AMIX, BMIX, and in addition AMIX_MAX and BMIX_MAX for the spin-polarized runs to improve convergence ) 
--) also, it may help to increase NELMDL (the number of non-selconsistent steps at the beginning) to improve the pre-convergence of the wavefunction 
--) maybe it also helps to use L(S)DA+U ( please check the electronic structure and compare to experiment, if results are available)

2) for the forces which suddenly increase: 
--) please check (in OSZICAR) if each ionic step was fully converged electronically up to the requested convergence limit (EDIFF) before the forces are calculated. (i.e. whether the forces were calculated after el. convergence is reached or because the number of electronic convergence steps (NELM=60, by default) was reached without actually being converged. The latter yields unreasonable forces. (in that case, please either increase NELM and/or try one of the above mentioned to improve the electronic convergence) 
--) if each of the ionic steps was converged, please check the relaxation history (XDATCAR file). Sometimes, especially the first ionic steps 'overshoot' a little. If that is the case, please choose a different relaxation algorithm (IBRION) and/or decrease the step width (POTIM)

3) the VERY BAD NEWS message is due to different lattice types obtained for the unit cell and the symmetrized k-mesh. (triclinic and simple monoclinic as it seems). maybe the axes ratios of the unit cells are not 1:1:1 ? please check. 
for the symmetry analysis, the 
Bravais Matrix , 
atoms' positions, and, if set, 
magnetic moments and 
atoms' velocities are taken into account. 
If nothing has been changed except the atoms in the PP, it seems strange that this warning shows up for some calculations whereas it does not for others. In any case, if ISYM=0 is set, symmetrization is not taken into account in the calculation later on, the only point to be careful with is the axis' ratio then.

LDAU = .TRUE.
LDAUL = 
LDAUU = 
LDAUJ =