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ISMEAR设置再谈

已有 37821 次阅读 2015-6-4 08:46 |个人分类:结构优化|系统分类:科研笔记

关注：

1) 结构弛豫时，不同的ISMEAR设置对晶格常数和总能的影响

2) 对于不同ISMEAR值，在比较大体系下快速检验SIGMA取值适当与否的途径


问题：

结构弛豫时ISMEAR参数的设置似乎会显著影响能量值，请问对不同高压相结构做能差计算时，是不是不能考虑金属、绝缘体差别，将ISMEAR参数统一设置为相同值？


for very accurate DOS and total energy calculations ISMEAR=-5 always should be used.
if the electronic structure changes from metallic to insulating upon
substitution of certain atoms, please do the following
1)
-- for the GEOMERTY relaxations of the METALLIC system, use
the Methfessel-Paxton method (ISMEAR =1 or 2)
-- (for the insulating phase, ISMEAR=-5 can be used for the geometry optimisation)

【两者能量比较，能得出真实能差吗？】

2) if the structures are fully relaxed, do one final (electronic scf) step with ISMEAR=-5 for the metallic phase as well, without any further change in the geometry.

3) the DOS should be obtained with Bloechl's method for both systems



引子：面心立方结构XH3优化过程，不同优化次数下，焓值迥异，能级占据诡异的解决方案  由ISMEAR = 1改为ISMEAR = -5【或许X的POTCAR根本就没有选好】


       ISMEAR = -5 保险起见设置成-5，如设置成1或0，有可能级占据数出错，从而多次优化后能量仍然相差很大。

ISMEAR=1时，面心立方结构XH3

OUTCAR_800-02:                  Total CPU time used (sec):     1385.830
OUTCAR_800-03:                  Total CPU time used (sec):     1759.621


774.22    100.48    -34.66049192   OUTCAR_800-01
774.27    100.47    -34.66053244   OUTCAR_800-04
800.07    100.47    -35.64301370   OUTCAR_800-03
800.35    100.62    -37.01398180   OUTCAR_800-02



CONTCAR_800-04


XH3                    
  1.00000000000000    
   4.6366048767523642   -0.0000000000000000    0.0000000000000000
    0.0000000000000000    4.6366048767523642   -0.0000000000000000
    0.0000000000000000   -0.0000000000000000    4.6366048767523642
  H    X

  12     4
Direct
 0.5000000000000000  0.5000000000000000  0.5000000000000000
 0.5000000000000000  0.0000000000000000  0.0000000000000000
 0.0000000000000000  0.5000000000000000  0.0000000000000000
 0.0000000000000000  0.0000000000000000  0.5000000000000000
 0.7500000000000000  0.2500000000000000  0.2500000000000000
 0.2500000000000000  0.7500000000000000  0.7500000000000000
 0.2500000000000000  0.7500000000000000  0.2500000000000000
 0.7500000000000000  0.2500000000000000  0.7500000000000000
 0.2500000000000000  0.2500000000000000  0.7500000000000000
 0.7500000000000000  0.7500000000000000  0.2500000000000000
 0.7500000000000000  0.7500000000000000  0.7500000000000000
 0.2500000000000000  0.2500000000000000  0.2500000000000000
 0.0000000000000000 -0.0000000000000000 -0.0000000000000000
 0.0000000000000000  0.5000000000000000  0.5000000000000000
 0.5000000000000000 -0.0000000000000000  0.5000000000000000
 0.5000000000000000  0.5000000000000000 -0.0000000000000000


ISMEAR=-5时，面心立方结构XH3

OUTCAR_800-01:                  Total CPU time used (sec):     1401.258
OUTCAR_800-02:                  Total CPU time used (sec):      340.458
OUTCAR_800-03:                  Total CPU time used (sec):      339.044
OUTCAR_800-04:                  Total CPU time used (sec):      339.415


799.92    100.45    -35.63488710   OUTCAR_800-01
800.08    100.45    -35.63699566   OUTCAR_800-02
800.08    100.45    -35.63699566   OUTCAR_800-03
800.08    100.45    -35.63699566   OUTCAR_800-04


CONTCAR_800-04

XH3                  
  1.00000000000000    
    5.1862242792515811   0.0000000000000000    0.0000000000000000
    0.0000000000000000    5.1862242792515802    0.0000000000000000
    0.0000000000000000    0.0000000000000000    5.1862242792515838
  H    X

  12     4
Direct
 0.5000000000000000  0.5000000000000000  0.5000000000000000
 0.5000000000000000  0.0000000000000000  0.0000000000000000
 0.0000000000000000  0.5000000000000000  0.0000000000000000
 0.0000000000000000  0.0000000000000000  0.5000000000000000
 0.7500000000000000  0.2500000000000000  0.2500000000000000
 0.2500000000000000  0.7500000000000000  0.7500000000000000
 0.2500000000000000  0.7500000000000000  0.2500000000000000
 0.7500000000000000  0.2500000000000000  0.7500000000000000
 0.2500000000000000  0.2500000000000000  0.7500000000000000
 0.7500000000000000  0.7500000000000000  0.2500000000000000
 0.7500000000000000  0.7500000000000000  0.7500000000000000
 0.2500000000000000  0.2500000000000000  0.2500000000000000
 0.0000000000000000  0.0000000000000000  0.0000000000000000
 0.0000000000000000  0.5000000000000000  0.5000000000000000
 0.5000000000000000  0.0000000000000000  0.5000000000000000
 0.5000000000000000  0.5000000000000000  0.0000000000000000


CONTCAR_800-04

XH3              
  1.00000000000000    
    4.6485804347186610   0.0000000000000000    0.0000000000000000
    0.0000000000000000    4.6485804347186610    0.0000000000000000
    0.0000000000000000    0.0000000000000000    4.6485804347186610
  H    X
   12     4
Direct
 0.5000000000000000  0.5000000000000000  0.5000000000000000
 0.5000000000000000  0.0000000000000000  0.0000000000000000
 0.0000000000000000  0.5000000000000000  0.0000000000000000
 0.0000000000000000  0.0000000000000000  0.5000000000000000
 0.7500000000000000  0.2500000000000000  0.2500000000000000
 0.2500000000000000  0.7500000000000000  0.7500000000000000
 0.2500000000000000  0.7500000000000000  0.2500000000000000
 0.7500000000000000  0.2500000000000000  0.7500000000000000
 0.2500000000000000  0.2500000000000000  0.7500000000000000
 0.7500000000000000  0.7500000000000000  0.2500000000000000
 0.7500000000000000  0.7500000000000000  0.7500000000000000
 0.2500000000000000  0.2500000000000000  0.2500000000000000
 0.0000000000000000  0.0000000000000000  0.0000000000000000
 0.0000000000000000  0.5000000000000000  0.5000000000000000
 0.5000000000000000  0.0000000000000000  0.5000000000000000
 0.5000000000000000  0.5000000000000000  0.0000000000000000



ISMEAR1-pot-s时能级占据情形   E-fermi :   7.9821

费米能级以上可以有态密度(DOS)，但费米能级以上的能级（ band energies  ）无电子占据(occupation=0;  band)



k-point   5 :       0.1667    0.1667    0.0000
 band No.  band energies     occupation
     1     -37.8249      1.00000
     2     -37.7737      1.00000
     3     -37.7737      1.00000
     4     -37.7700      1.00000
     5     -11.4911      1.00000
   .....
    16     -10.5205      1.00000
    17       0.1417      1.00000
    18       3.3491      1.00000
 ......
    48       7.7715      1.02978     E-fermi :   7.9821
    49       8.3008     -0.02337
    50       8.3896     -0.00706
    51       8.3896     -0.00706
    52       9.6448     -0.00000
    53      10.0274     -0.00000
    54      10.1542     -0.00000
    55      10.4690     -0.00000
    56      10.6724     -0.00000




ismear=-5,pot
E-fermi :   7.8983


k-point   5 :       0.1667    0.1667    0.0000
 band No.  band energies     occupation
     1     -37.9092      1.00000
     2     -37.8575      1.00000
     3     -37.8575      1.00000
     4     -37.8537      1.00000
  ...
    16     -10.5788      1.00000
    17       0.0677      1.00000
    18       3.2780      1.00000
  .......
    48       7.6905      1.02884    E-fermi :   7.8983
    49       8.2238     -0.02188
    50       8.3126     -0.00633
    51       8.3126     -0.00633
    52       9.5853     -0.00000
    53       9.9800     -0.00000
    54      10.0872     -0.00000
    55      10.4160     -0.00000
    56      10.5923     -0.00000


     

ISMEAR=-5, pot,80GPa    【考虑到费米能级后，能级占据并未出现异常】
E-fermi :  14.1773
 
k-point   5 :       0.1667    0.1667    0.0000
 band No.  band energies     occupation
     1     -32.5027      1.00000
     2     -32.1734      1.00000
   ....
    16      -4.3574      1.00000
    17       4.1130      1.00000
    18       6.3884      1.00000
    19       6.6324      1.00000
   ......
    28      11.7277      1.00000
    ......
    45      13.8867      1.03395
    46      14.0076      0.96899
    47      14.0076      0.96899
    48      14.0091      0.96645
    49      14.0091      0.96645
    50      14.0750      0.80364
    51      14.1123      0.66827      E-fermi :  14.1773
    52      14.5266     -0.01195
    53      15.2285     -0.00000
    54      15.2573     -0.00000
    55      15.5089     -0.00000
    56      15.5823     -0.00000

                                             

                               




手册解读：

http://cms.mpi.univie.ac.at/vasp/vasp/ISMEAR_SIGMA_FERWE_FERDO_SMEARINGS_tag.html#3951


Default  
ISMEAR=1
SIGMA=0.2

 

1..$ N$method of  Methfessel-Paxton order $ N$.
Mind: For the Methfessel-Paxton scheme the partial occupancies  can be negative


$ -5$tetrahedron method with Blöchl corrections (use a $ .Gamma $-centered k-mesh, see sec.5.5 )

For the calculation of the total energy in bulk materials we recommend the   tetrahedron method with Blöchl corrections (ISMEAR=-5). 

This method also gives a good account for the  electronic density of states (DOS).

The only drawback is that the methods is not variational with respect to the partial occupancies.  Therefore the calculated forces and the stress tensor  can be wrong by up to 5 to 10 % for metals.  For the calculation of phonon frequencies based on forces we recommend the method of  Methfessel-Paxton (ISMEAR$ >$0). For semiconductors and insulators the forces are correct, because partial occupancies do not vary and are zero or one.


The method of   Methfessel-Paxton (MP) also results in a very accurate  description of the total energy, nevertheless the width of the smearing (SIGMA) must be chosen carefully (see also 7.4). 

Too large smearing-parameters might result in a wrong total energy, small smearing parameters require a large k-point mesh. SIGMA should be as large as possible keeping the difference between the free energy and the total energy  (i.e. the term 'entropy T*S') in the OUTCAR file negligible (1 meV/atom). In most cases  $ N=1$ and $ N=2$ leads to very similar results. The method of MP is also the method of choice for large supercells, since the  tetrahedron method is not applicable, if less than three k-points are used.  

Mind: Avoid using ISMEAR$ >$0 for semiconductors and insulators, since this often leads to incorrect results (The occupancies of some states might be larger or smaller than 1).  For insulators use ISMEAR=0 or ISMEAR=-5.



For further considerations on the choice for the smearing method see sections  7.4,8.6. To summarize,  use the following guidelines:

  • For semiconductors or insulators use the tetrahedron  method (ISMEAR=-5), if the cell is too large (or if you use only a single or two k-points) use ISMEAR=0 in combination with a small SIGMA=0.05.




  • For relaxations in metals  always use ISMEAR=1 or  ISMEAR=2 and an appropriate SIGMA value (the entropy term should be less than 1 meV per atom). Mind: Avoid to use ISMEAR$ >$0 for semiconductors and insulators, since it might cause problems.  

    For metals a sensible value  is usually SIGMA= 0.2 (which is the default).






  • For the calculations of the DOS and very accurate  total energy calculations (no relaxation in metals) use the tetrahedron method (ISMEAR=-5).

【对金属体系的弛豫不能用ISMEAR=-5，绝缘体结构的弛豫能用ISMEAR$ >$0 吗？】


摘录学习：

Role of ISMEAR in insulator-conductor transition

http://blog.sciencenet.cn/blog-478347-367187.html

 

小喽喽问：

I am working now with a transition metal oxides. When I change some oxygens and put nitrogen in its place, the electronic structure change from insulator to conductor.

    For total energy insulator's bulk calculations I have used ISMEAR=-5 (tetrahedron method with Blöchl corrections), but when the solid become conductor I must use ISMEAR=1 or 2.

My question is: Do I must compare the energies got with diferent ISMEAR keeping the same all others parameters?

Other question related: Do I must get the DOS and total energy for a optimized conductor structure whih ISMEAR=-5 or with ISMEAR=1 or 2?

Other question related: I get occupancies of some states larger than 2 with ISMEAR>0 for conductors too. Then, What are exactly the problems derivated from to use ISMEAR > or = 0 for insulators and semiconductors?

Thak you so much

大牛答：

for very accurate DOS and total energy calculations ISMEAR=-5 always should be used.
if the electronic structure changes from metallic to insulating upon
substitution of certain atoms, please do the following
1)
-- for the GEOMERTY relaxations of the METALLIC system, use
the Methfessel-Paxton method (ISMEAR =1 or 2)
-- (for the insulating phase, ISMEAR=-5 can be used for the geometry optimisation)

2) if the structures are fully relaxed, do one final (electronic scf) step with ISMEAR=-5 for the metallic phase as well, without any further change in the geometry.

3) the DOS should be obtained with Bloechl's method for both systems


 

ISMEAR是决定电子在费米面附近怎么填充的，电子的填充决定总能量，总能量对位移的导数是原子受到的力，结构够优化是跟据原子受到的力一点点优化的，直到受到的的力小于收敛条件，所以ISMEAR在结构优化过程也要设置。


kmw.8668(站内联系TA)

ISMEAR决定了如何确定每个波函数的占有数 ，驰豫计算时一般不需要这个参数。
进行任何的静态计算或态密度计算，且K点数目（从IBZKPT文件中读取）大于4时，取ISMEAR＝－5；当由于原胞较大而K点数目较少（小于4个）时，取ISMEAR＝0，并设置一个合适的SIGMA值。另外对半导体或绝缘体的计算（不论是静态还是结构优化），取ISMEAR＝－5;当体系呈现金属性时，取ISMEAR＝1和2，以及设置一个合适的SIGMA值。在进行能带结构计算时，ISMEAR 和SIGMA用默认值就好。一般说来，无论是对何种体系，进行何种性质的计算，采用ISMEAR＝0 ，并选择一个合适的SIGMA值都能得到合理的结果。


先用几个离子步(NSW=10)来跑一下看看.对于不熟悉的体系,我先用ISMEAR=-5跑十步,看看输出的提示和消耗情况,然后来调整参数.自然ISMEAR,SIGMA,POTIM都是调节的对象.SIGMA=0(ncg~=900)比-5(ncg~=4000)跑的快的多.
但是我不明白为什么.




 

晶格优化的结果很诡异

http://emuch.net/html/201010/2455996.html

我用侯柱峰在《VASP使用指南入门》中的“复杂的情况，以六角结构Mg的晶格常数为例”做的结构优化，不论我取值范围是多少，计算得到的能量在我取值范围内都是单调增的（所以每一次更换取值范围时，用murn得到的“平衡体积”对应的晶格轴参数都不一样）。我用的run_cell文件：

你计算的体系，晶格常数有三个变量(a,b,c，即它们是不等的),另外还有原子的位置需要优化。因此为了得到状态曲线，需采用ISIF=4对一系列体积点进行固定体积的变原胞形状以及原子位置的优化。因此做run_cell脚本里面做relaxation时的输入文件可设置为：
System = CaFe2As2 afm1-opt
PREC = Accurate
ISPIN = 2
MAGMOM = 4*0 3 3 -3 -3 3 -3 -3 3 8*0
ISMEAR = 1
SIGMA = 0.20
ISTART = 0
EDIFF = 1E-5
EDIFFG = -1E-3
ICHARG = 2
ENCUT = 300
NSW = 60
IBRION = 2
ISIF = 4
POTIM = 0.2
#LORBIT = 11
LREAL = Auto
#LPLANE = .TRUE.
#LSCALU = .FALSE.
#PAR = 4
#SIM = 4
在特到了一些体积点对应的总能，及 V----E的数据点，采用状态方程拟合后，得到平衡体积，再采用run_cell里面的初始b和c（这里假定run_cell里面的循环变量是对晶格常数a的）由V_0 =  a*b*c得到这个平衡体积下 a, ，然后再做一次ISIF=4固定体积，变原胞和优化原子位置的结构优化。
原来的例子中是对六角晶体的Mg进行计算，该体系只需要优化晶格形状就可以了，不需要优化原子位置，因此可以采用ISIF=5。
你贴出来的结果显示，平衡体积还在303.1000的值以下，还得做一些体积值比303.1000小的体积的计算。

 


关于vasp中ISMEAR参数的几个问题

http://emuch.net/html/201212/3954918.html


对于ISMEAR参数有两个问题向大家咨询：
1.ISMEAR设置，看了几篇关于MC结构的文献，没明确说到其在结构弛豫时ISMEAR值，因此向大家求助，对于像MC体系结构的弛豫与静态计算的ISMEAR一般设置多少？另外，对于不同ISMEAR值，在比较大体系下有没什么快速检验SIGMA取值适当与否的途径？


2.假设对于绝缘体，取ISMEAR＝1，与ISMEAR＝-5有多大区别？手册上写的是：For relaxations in metals always use ISMEAR＝1 or 2, and a appropriated SIGMA value.


Mind: Avoid to use ISMEAR>0 for semiconductors and insulators, since it might cuase problem.


期待大家的解答！


就像手册上写的那样，ISMEAR=1或2时，是针对于金属而言的，对于绝缘体或半导体来说一般选取的是0或-5,而取-5时候，主要是能量值得差异，其TOTEN 和energy without entropy的值是一样的~【不一样哈？ --Ye】


vasp算的能量相加减是否需要设置相同的ENCUT和ISMEAR以及SIGMA?

在用vasp计算表面吸附时，算吸附能，是否要在金属表面和吸附气体INCAR中采用同一个ENCUT和ISMEAR以及SIGMA？还有其他需要不变的量吗？

参数必须一致，而你的这些参数对能量的影响还都挺大的



vasp 输入文件中的ISMEAR参数指的是波函数占据数目，但是这个到底是什么意思？可以浅显一点讲吗？


就是说电子在费米面附近占据数从0突变到1，这是个deta函数，为了计算方便，用一个平滑点的函数在费米面附近代替这个deta函数，这样计算就不容易振荡，易于收敛。ismear就是控制这种平滑函数的。


http://baike.baidu.com/link?url=QoiVmYpUsK0mvBEqmYWtph7X1hrY4X5WGbX7I2Wpsui40q-PpXEfIqAqHxfuOWojHgNW4b5dvvI6kh9XC5_DNa

提示：在计算能带结构时，采用ISMEAR = 0或1对结果的影响非常小，可以认为是一样的。但是不能采用ISMEAR = -5 或-4。








https://blog.sciencenet.cn/blog-567091-895413.html

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