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VASP旋轨耦合的设置初探

已有 42072 次阅读 2014-5-22 15:06 |个人分类:电子结构计算|系统分类:科研笔记|关键词:学者

关注：

1）旋轨耦合的物理含义及参数设置

 

 

 

网络摘录：

http://blog.sciencenet.cn/home.php?mod=spacecp&ac=blog

 

 

1， vasp soc计算
a）非自洽计算SOC
     首先进行自旋极化计算（当然，如果体系没有磁矩，如Bi2Se3，初始磁矩取为零）。
     然后进行非自洽SOC计算：
              ICHARG = 11
       
               Spin orbit
               LSORBIT = .TRUE.
              SAXIS   =  0 0 1   ：因为之前进行了自旋极化计算，这里不需要重新指定MAGMOM。
              NBANDS  =  160 ：vasp要求NBNADS在自旋极化计算和soc计算时有精确两倍关系。
b) 全自洽计算SOC

2， vasp 考察体系的dipole potential
     在做全自洽计算时，打开选项
      LVTOT = T
   
      此后会生成LOCPOT文件。此文件可以用下面小程序读取

 

摘录：http://blog.sciencenet.cn/blog-588243-486029.html

将VASP 的makefile 文件中的 CPP 选项中的 -DNGXhalf, -DNGZhalf, -DwNGXhalf, -DwNGZhalf 这4个选项去掉重新编译VASP才能计算自旋轨道耦合效应。

 

以下是从VASP在线说明书整理出来的非线性磁矩和自旋轨道耦合的计算说明。          

非线性磁矩计算：

1）计算非磁性基态产生WAVECAR和CHGCAR文件。

2）然后INCAR中加上

ISPIN=2

ICHARG=1 或 11  ！读取WAVECAR和CHGCAR文件

LNONCOLLINEAR=.TRUE.

        MAGMOM=

注意：①对于非线性磁矩计算，要在x, y 和 z方向分别加上磁矩，如

MAGMOM = 1 0 0  0 1 0   ！表示第一个原子在x方向，第二个原子的y方向有磁矩

②在任何时候，指定MAGMOM值的前提是ICHARG=2（没有WAVECAR和CHGCAR文件）或者ICHARG=1 或 11（有WAVECAR和CHGCAR文件），但是前一步的计算是非磁性的（ISPIN=1）。

 

磁各向异性能（自旋轨道耦合）计算：

注意： LSORBIT=.TRUE. 会自动打开LNONCOLLINEAR= .TRUE.选项，且自旋轨道计算只适用于PAW赝势，不适于超软赝势。

自旋轨道耦合效应就意味着能量对磁矩的方向存在依赖，即存在磁各向异性能（MAE），所以要定义初始磁矩的方向。如下：

LSORBIT = .TRUE.

SAXIS = s_x s_y s_z (quantisation axis for spin)

默认值： SAXIS=(0+,0,1)，即x方向有正的无限小的磁矩，Z方向有磁矩。

 

要使初始的磁矩方向平行于选定方向，有以下两种方法：

MAGMOM = x y z ! local magnetic moment in x,y,z

SAXIS = 0 0 1 ! quantisation axis parallel to z

or

MAGMOM = 0 0 total_magnetic_moment ! local magnetic moment parallel to SAXIS （注意每个原子分别指定）

SAXIS = x y z !quantisation axis parallel to vector (x,y,z)，如 0 0 1 

两种方法原则上应该是等价的，但是实际上第二种方法更精确。第二种方法允许读取已存在的WAVECAR（来自线性或者非磁性计算）文件，并且继续另一个自旋方向的计算（改变SAXIS 值而MAGMOM保持不变）。当读取一个非线性磁矩计算的WAVECAR时，自旋方向会指定平行于SAXIS。

 

计算磁各向异性的推荐步骤是：

1）首先计算线性磁矩以产生WAVECAR 和 CHGCAR文件（注意加入LMAXMIX）。

2）然后INCAR中加入：

LSORBIT = .TRUE.

ICHARG = 11           ! non selfconsistent run, read CHGCAR

                                 !或 ICHARG ==1 优化到易磁化轴，但此时应提高EDIFF的精度

LMAXMIX = 4         ! for d elements increase LMAXMIX to 4, f: LMAXMIX = 6

                                   ! you need to set LMAXMIX already in the collinear calculation

SAXIS = x y z              ! direction of the magnetic field  如 0 0  1

NBANDS = 2 * number of bands of collinear run                 ！ grep NBANDS OUTCAR

ISYM=0                         ！switch off symmetry (ISYM=0) when spin orbit coupling is selected

GGA_COMPAT=.FALSE.              ！ it improves the numerical precision of GGA for non collinear calculations

  LORBMOM=.TRUE.                       !计算轨道磁矩

 继续计算，VASP会读取WAVECAR 和 CHGCAR将自旋量子化方向（磁场方向）平行于SAXIS方向。

最后可以比较各个方向磁矩时能量的不同。

注意： 第二步使用自洽计算(ICHARG=1)原则上也是可以的，但是初始平行于SAXIS的磁场发生旋转，直到达到基态，如平行于易磁化轴，但这个过程会很慢且能量变化很小，而且如果收敛标准不是很严格的话，自洽计算会在未达到基态就停止。

 

注意： VASP的输入输出的磁矩和类自旋量都会按照这个SAXIS方向，包括INCAR中的

 MAGMOM行，OUTCAR和PROCAR文件中的总磁矩和局域磁矩，WAVECAR中的类自旋轨道和CHGCAR中的磁性密度。 With respect to the cartesian lattice vectors the components of the magnetization are (internally) given by    

.begin{eqnarray*} m_x & = & .cos(.beta) .cos(.alpha) m^{.rm axis}_x- .sin(.alph... ..._z & = & -.sin(.beta) m^{.rm axis}_x+ .cos(.beta) m^{.rm axis}_z .end{eqnarray*}Where $m^{.rm axis}$ is the externally visible magnetic moment. Here, $.alpha$ is the angle between the SAXIS vector $(s_x,s_y,s_z)$ and the cartesian vector $.hat x$, and $.beta$ is the angle between the vector SAXIS and the cartesian vector $.hat z$:.begin{eqnarray*} .alpha &=& {.rm atan} .frac{s_y}{s_x} .. .beta &=& {.rm atan} .frac{.vert s_x^2+s_y^2.vert}{s_z} .end{eqnarray*}

 

The inverse transformation is given by  

 

.begin{eqnarray*} m^{.rm axis}_x & = & .cos(.beta) .cos(.alpha) m_x + .cos(.bet... ... cos(.alpha) m_x+ .sin(.beta) .sin(.alpha) m_y + .cos(.beta) m_z .end{eqnarray*}

 

It is easy to see that for the default $(s_x, s_y, s_z)=(0+,0,1)$, both angles are zero, i.e. $.beta=0$ and $.alpha=0$. In this case, the internal representation is simply equivalent to the external representation:  

 

.begin{eqnarray*} m_x & = & m^{.rm axis}_x .. m_y & = & m^{.rm axis}_y .. m_z & = & m^{.rm axis}_z .end{eqnarray*}

 

The second important case, is $m^{.rm axis}_x=0$ and $m^{.rm axis}_y=0$. In this case  

 

.begin{eqnarray*} m_x & = &.sin(.beta)*.cos(.alpha) m^{.rm axis}_z = m^{.rm axi... ...^{.rm axis}_z = m^{.rm axis}_z s_z / .sqrt{s_x^2+s_y^2+s_z^2} .end{eqnarray*}

 

Hence now the magnetic moment is parallel to the vector SAXIS. Thus there are two ways to rotate the spins in an arbitrary direction, either by changing the initial magnetic moments MAGMOM or by changing SAXIS. 


      MAGMOM-tag：http://cms.mpi.univie.ac.at/vasp/vasp/MAGMOM_tag.html#incar-magmom

      LNONCOLLINEAR-tag： http://cms.mpi.univie.ac.at/vasp/vasp/LNONCOLLINEAR_tag.html

LSORBIT-tag http://cms.mpi.univie.ac.at/vasp/vasp/LSORBIT_tag.html

 

 

问题： 第一步线性计算得到WAVECAR 和 CHGCAR文件，必须是静态计算的WAVECAR 和 CHGCAR文件吗？ 动态优化的可不可以？静态计算需要使用NUPDOWN 锁定磁矩吗？

      进行非线性磁矩或自旋轨道耦合计算的时候，结构需不需要重新优化？

我现在的做法是： 先加入LMAXMIX = 4结构优化，然后仍然使用LMAXMIX = 4静态计算（ICHARG=2，LWAVE=.TRUE.，LCHARG=.TRUE.），然后进行高收敛标准的静态的soc自洽计算来考虑soc的影响，也不知对不对。

 

 

 

 

 

 

摘录2

http://emuch.net/html/201201/3988356.html

参照说明书上的做法
1、无soc，线性计算。得到WAVECAR, CHG, CHGCAR
2、非线性计算，加入SOC，
如果再更改KPOINTS，像以往的能带计算一样，就会有错误。

所以想求教各位，对于这样的体系，怎样求得加入SOC后的能带结构。

 

fzx2008(站内联系TA)

我在计算时，只在计算能带时加入了SOC的关键字做非共线计算
1.常规DFT优化
2.共线SCF，输出CHGCAR
3.读取上一步电荷密度，指定K点路径，非共线计算
注：SCF时就想考虑SOC，可以读取上一步波函数，并把上一步的IBZKPT拷贝成KPOINTS，以保持k点一致

 

cenwanglai(站内联系TA)

那这么说稀土元素都要考虑了？但是我见过PRB上忽略Er的自旋轨道耦合的。
加入自旋轨道耦合在计算量上影响大不大？
自旋轨道耦合除了对能带结构和禁带宽度影响较大外，对一般总能和几何结构影响大不大呢？

 

fzx2008(站内联系TA)

固体计算中，稀土元素较少考虑SOC的。这儿的难点基本在4f电子的处理，采用+U或者PBE0及类似
加入SOC常常要关闭对称性，计算量会大些
SOC对几何结构的影响较小，至少我测试的这些体系来说。欢迎各位补充。

 

拓朴绝缘体结构在优化阶段有需要考虑soc嘛？
还是说只要考虑普通的自旋极化即可
这会不会对之后的结果产生很大的偏差？

 

 

摘录：http://emuch.net/html/201311/6643193.html

各位前辈，我在用VASP做自旋轨道耦合时，提示错误：Error reading item 'MAGMOM' from file INCAR.
Error code was IERR=0 ... . Found N=  124 data.
我的INCAR如下：
SYSTEM=La2CoMnO6
ENCUT=550
ISTART=1
ICHARG=11
ISMEAR=-5
PREC=low
ISPIN=2
#IBRION=2
#POTIM=0.2
#ISIF=3
NELM=300
#NSW=200
LORBIT=11
LDAU=.TRUE.
LDAUL=-1  2 2 -1
LDAUU=0.5 5 5 0.5
LDAUJ=0  1  1  0
#LAECHG=.TRUE.
RWIGS=0.820 1.323 1.302 1.535
LSORBIT=.TRUE.
SAXIS=0 0 1
NBANDS=784
#LMAXMIX=1 4 4 1
MAGMOM=0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
初次算这个，请各位赐教。谢谢。

 

MAGMOM 是給 initial moment
有多少原子就要給多少數字
但這個參數是有長度限制
所以原子太多~ MAGMOM 寫得太長的話
就會出問題
最簡單的解決之道就是減少數字的出現
比方你有一百顆原子
每個 initial moment 都是 0 的話
那就寫成 MAGMOM = 100*0
這樣就可以有 100 個 0 了

 

MAGMOM设置时，要续行的话，用 \符号（反斜杠符号）。

 

我按照您的提示改了下INCAR：SYSTEM=La2CoMnO6
ENCUT=550
ISTART=1
ICHARG=11
ISMEAR=-5
PREC=low
ISPIN=2
#IBRION=2
#POTIM=0.2
#ISIF=3
NELM=300
#NSW=200
LORBIT=11
LDAU=.TRUE.
LDAUL=-1  2 2 -1
LDAUU=0.5 5 5 0.5
LDAUJ=0  1  1  0
#LAECHG=.TRUE.
RWIGS=0.820 1.323 1.302 1.535
LSORBIT=.TRUE.
SAXIS=0 0 1
NBANDS=784
#LMAXMIX=1 4 4 1
MAGMOM=47*0 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 16*0
但是还是提示错误：Error reading item 'MAGMOM' from file INCAR.
Error code was IERR=0 ... . Found N=  111 data.
而且正好写完一行，不用换行。

 

anewtry(站内联系TA)

 

哦，我之前自洽时直接设置的ISPIN=2磁矩，磁矩设置是MAGMOM= 48*0.5 8*5 8*5 16*0.5。您能说说用VASP做自旋轨道耦合具体的步骤吗？我看说明手册做的，

先在自旋情况下优化结构，然后进行自洽计算，得到CHGCAR和WAVECAR，

然后加入LSORBIT=.TRUE.
SAXIS=0 0 1
NBANDS=784
MAGMOM=47*0 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 0 0 5 16*0
这些项，进行非自洽的计算。不知道这样对不对？

 

sxjsn1(站内联系TA)

你这有点乱。。。  先自洽计算得到晶格结构（第一次计算）
然后使用优化后的POSCAR，新的INCAR,KPONTS和POTCAR重新计算（第二次计算）。
这时INCAR中LSORBIT=.T（开自旋极化）， 此时的磁矩，每三个数据为一个原子三个轴方向的磁矩分量。 比如，MAGMOM=0 0 5，表示一个原子x方向磁矩0，y方向磁矩0，z方向磁矩5。
设置好后，进行计算，得到CHAGCAR.
然后，你再用这个CHAGCAR计算band和DOS。
看你主楼的INCAR，ICHARG=11,这种情况下是调用已有的CHAGCAR文件。你如果想产生新的CHAGCAR文件，这个参数需删掉。

 

anewtry(站内联系TA)

嗯，是79个，47+8+8+16.
我昨天又看了下说明书，上面说非线性的磁性，第一步先进行非磁性的结构优化，得到CHGCAR和WAVECAR。然后读得到的CHGCAR和WAVECAR，并加旋轨耦合的参数进行自洽，最后再算非自洽的band和dos.
我一开始使用ISPIN=2优化的，所以我觉得不太对，就想是不是先ISPIN=1 优化下，再试试。
还有就算是，说明书上给MAGMOM是000 000 001 这样给的。我在想我算的前面47 和后面16个磁矩为0 的MAGMOM能直接47*000 和16*000吗？请指教，谢谢

 

摘录3   VASP考虑自旋轨道耦合的话，如何编译

http://emuch.net/html/201007/2177790.html

 

 

就这么简单？
为什么我从VASP论坛上看到这样的表述：
If you include relativistic effects (LSORBIT), non-collinear magnetism is automatically included as well (if you have not set LNONCOLLINEAR = .True. yourself). The appropriate vasp-executable for non-collinear magnetic structures has to be generated without the -DNGXhalf,... -DwNGXhalf,... precompiler options

楼上的可能误导你了，如果算自选轨道耦合，一定要重新编译vasp，编译很简单,只要去掉makefile中-DNGXhalf, -DNGZhalf, -DwNGXhalf, -DwNGZhalf 这4个选项就可以了。否则你计算自选轨道耦合就会提示出错。你可以试试看。
还有计算自选轨道耦合一般是计算magnetocrystalline anisotropy. 和是不是d,f轨道没有关系。
我们常说计算体系有d,f 轨道是要考虑有没有强关联，在vasp里面就是考虑要不要加U.

灯塔守望者(站内联系TA)

sunray55是对的，我记错了。
把65行（行号未必严格对上）及以下几行内容改为：
CPP     = $(CPP_)  -DHOST=\"LinuxIFC\" \
          -Dkind8 -DNGXhalf -DCACHE_SIZE=12000 -DPGF90 -Davoidalloc \
          -Duse_cray_ptr
#          -DRPROMU_DGEMV  -DRACCMU_DGEMV
把185行及以下几行内容改为：
CPP    = $(CPP_) -DMPI  -DHOST=\"LinuxIFC\" -DIFC \
     -Dkind8 -DNGZhalf -DCACHE_SIZE=4000 -DPGF90 -Davoidalloc \
     -DMPI_BLOCK=2000  \
     -Duse_cray_ptr    \

     -DRPROMU_DGEMV  -DRACCMU_DGEMV

 

 

摘录：


可以看看这篇博文
LSORBIT = .TRUE.
GGA_COMPAT=.FALSE.
ISYM=-1
LORBMOM=.TRUE.
MAGMOM= 给定一个初始磁矩
LNONCOLLINEAR=.TRUE.  #其实指定了LSORBIT之后，这个参数就默认打开了
这些参数指定了，其实就已经是非共线计算了。
输出的数据包含magnetization (x)
magnetization (y)
magnetization (z)对应POSCAR中的各个原子，
这个分量每5个scf loop更新一次，可以在OUTCAR中查看
此外，我觉得，没必要按照手册先计算non-magnetic计算，然后读入波函数再计算非共线，其实可以直接非共线跑的，省去那个所谓的non-magnetic计算其实可以减少很多麻烦，我们这都是直接这么做得。。。

 

手册介绍：

LSORBIT-tag

Supported as of VASP.4.5.

LSORBIT=.TRUE. switches on spin-orbit coupling and automatically sets  LNONCOLLINEAR= .TRUE.. This option works only for PAW potentials and is not supported for ultrasoft pseudopotentials.

 

If spin-orbit coupling is not included, the energy does not depend on the direction of the magnetic moment, i.e. rotating all magnetic moments by the same  angle results exactly in the same energy.

    Hence there is no need to define the spin quantization axis, as long as spin-orbit coupling is not included.

 

Spin-orbit coupling, however, couples the spin to the crystal structure. Spin orbit coupling is switched on by selecting   LSORBIT = .TRUE.  SAXIS =   s_x s_y s_z (quantisation axis for spin)  GGA_COMPAT = .FALSE. ! apply spherical cutoff on gradient field

where the default for  SAXIS=$ (0+,0,1)$ (the notation $ 0+$ implies an infinitesimal small positive number  in $ .hat x$ direction).  

 

The flag GGA_COMPAT (see Sec. 6.42)  is optional and should be set when small energy differences in the sub meV regime need to be calculated (often the case for magnetic anisotropy calculations). All magnetic moments are now given with respect to the axis $ (s_x,s_y,s_z)$, where we have adopted the convention that all magnetic moments and spinor-like quantities written or read by VASP are given with respect to this axis.

 

This includes the MAGMOM line in the INCAR file, the total and local magnetizations in the OUTCAR and PROCAR file, the spinor-like orbitals in the WAVECAR file, and the magnetization density in the CHGCAR file. With respect to the cartesian lattice vectors the components of the magnetization are (internally) given by

$.displaystyle m_x$$.displaystyle =$$.displaystyle .cos(.beta) .cos(.alpha) m^{.rm axis}_x- .sin(.alpha) m^{.rm axis}_y+ .sin(.beta)*.cos(.alpha) m^{.rm axis}_z$ 
$.displaystyle m_y$$.displaystyle =$$.displaystyle .cos(.beta) .sin(.alpha) m_x + .cos(.alpha) m^{.rm axis}_y + .sin(.beta) .sin(.alpha) m^{.rm axis}_z$ 
$.displaystyle m_z$$.displaystyle =$$.displaystyle -.sin(.beta) m^{.rm axis}_x+ .cos(.beta) m^{.rm axis}_z$ 


Where  $ m^{.rm axis}$ is the externally visible magnetic moment. Here, $ .alpha$ is the angle between the SAXIS vector  $ (s_x,s_y,s_z)$ and the cartesian vector $ .hat x$,  and $ .beta$ is the angle between the vector SAXIS and the cartesian vector $ .hat z$:

$.displaystyle .alpha$$.displaystyle =$$.displaystyle {.rm atan} .frac{s_y}{s_x}$ 
$.displaystyle .beta$$.displaystyle =$$.displaystyle {.rm atan} .frac{.vert s_x^2+s_y^2.vert}{s_z}$ 


The inverse transformation is given by

$.displaystyle m^{.rm axis}_x$$.displaystyle =$$.displaystyle .cos(.beta) .cos(.alpha) m_x + .cos(.beta) .sin(.alpha) m_y + .sin(.beta) m_z$ 
$.displaystyle m^{.rm axis}_y$$.displaystyle =$$.displaystyle - sin(.alpha) m_x + .cos(.alpha) m_y$ 
$.displaystyle m^{.rm axis}_z$$.displaystyle =$$.displaystyle .sin(.beta) cos(.alpha) m_x+ .sin(.beta) .sin(.alpha) m_y + .cos(.beta) m_z$ 


It is easy to see that for  the default $ (s_x, s_y, s_z)=(0+,0,1)$, both angles are zero, i.e.$ .beta=0$ and $ .alpha=0$.  In this case,  the internal representation  is simply equivalent to the external representation:

$.displaystyle m_x$$.displaystyle =$$.displaystyle m^{.rm axis}_x$ 
$.displaystyle m_y$$.displaystyle =$$.displaystyle m^{.rm axis}_y$ 
$.displaystyle m_z$$.displaystyle =$$.displaystyle m^{.rm axis}_z$ 


The second important case, is  $ m^{.rm axis}_x=0$ and $ m^{.rm axis}_y=0$. In this case

$.displaystyle m_x$$.displaystyle =$$.displaystyle .sin(.beta)*.cos(.alpha) m^{.rm axis}_z = m^{.rm axis}_z s_x / .sqrt{s_x^2+s_y^2+s_z^2}$ 
$.displaystyle m_y$$.displaystyle =$$.displaystyle .sin(.beta) .sin(.alpha) m^{.rm axis}_z = m^{.rm axis}_z s_y / .sqrt{s_x^2+s_y^2+s_z^2}$ 
$.displaystyle m_z$$.displaystyle =$$.displaystyle .cos(.beta) m^{.rm axis}_z = m^{.rm axis}_z s_z / .sqrt{s_x^2+s_y^2+s_z^2}$ 


Hence now the magnetic moment is parallel to the vector SAXIS. Thus there are two ways to rotate the spins in an arbitrary direction, either by changing the initial magnetic moments MAGMOM or by changing SAXIS.

To initialise calculations with the magnetic moment  parallel to a chosen vector $ (x,y,z)$, it is therefore possible to either specify (assuming a single atom in the cell)  MAGMOM = x y z   ! local magnetic moment in x,y,z SAXIS =  0 0 1   ! quantisation axis parallel to zor   MAGMOM = 0 0 total_magnetic_moment   ! local magnetic moment parallel to SAXIS SAXIS =  x y z   ! quantisation axis parallel to vector (x,y,z)Both setups should in principle yield exactly the same energy, but for implementation reasons the second method is usually more precise. The second method also allows to read a preexisting WAVECAR file (from a collinear or non collinear run), and to continue the calculation with a different spin orientation. When a non collinear WAVECAR file is read, the spin is assumed to be parallel to  SAXIS (hence VASP will initially report a magnetic moment in the z-direction only).

The recommended procedure for the calculation of magnetic anisotropies is therefore (please check the section on LMAXMIX6.63):

  • Start with a collinear calculation and calculate a WAVECAR and CHGCAR file.

  • Add the tags  LSORBIT = .TRUE. ICHARG = 11      ! non selfconsistent run, read CHGCAR LMAXMIX = 4      ! for d elements increase LMAXMIX to 4, f: LMAXMIX = 6  ! you need to set LMAXMIX already in the collinear calculation SAXIS =  x y z   ! direction of the magnetic field NBANDS = 2 * number of bands of collinear run GGA_COMPAT = .FALSE. ! apply spherical cutoff on gradient fieldVASP reads in the WAVECAR and CHGCAR files, aligns the spin quantization axis parallel to SAXIS, which implies that the magnetic field is now parallel to SAXIS, and performs a non selfconsistent calculation. By comparing the energies for different orientations the magnetic anisotropy can be determined. Please mind, that a completely selfconsistent calculation (ICHARG=1) is in principle also possible with VASP, but this would allow the the spinor wavefunctions to rotate from their initial orientation  parallel to SAXIS until the correct groundstate is obtained, i.e. until the magnetic moment is parallel to the easy axis. In practice this rotation will be slow, since reorientation of the spin gains little energy. Therefore if the convergence criterion is not too tight, sensible results might be obtained even for fully selfconsistent calculations (in the few cases we have tried selfconsistentcy worked without problems).

     

     

  • Be very careful with symmetry. We recommend to switch off symmetry (ISYM=0) altogether, when spin orbit coupling is selected. Often the k-point set changes from one to the other spin orientation, worsening the transferability of the results  (also the WAVECAR file can not be reread properly if the number of k-points changes). The flag GGA_COMPAT is usually required and should be set, since magnetic anisotropy energies are often in the sub meV regime (see Sec. 6.42).

  • Generally be extremely careful, when using spin orbit coupling and, specifically, magnetic anisotropies: energy differences are tiny, k-point convergence is tedious and slow, and the computer time might be huge. Additionally, this feature-- although long implemented in VASP-- is still in a late beta stage, as you might deduce from the frequent updates. No promise,  that your results will be useful! Here is a small summary from the README file:

    • 20.11.2003: The present GGA routine breaks the symmetry slightly for non orthorhombic cells.               A spherical cutoff is now imposed on the gradients and all intermediate results               in reciprocal space.               This changes the GGA results slightly (usually by 0.1 meV per atom), but is important               for magnetic anisotropies.

    • 05.12.2003:  continue... Now VASP.4.6 defaults to the old behavior GGA_COMPAT=.TRUE.,               the new behavior can be obtained by setting GGA_COMPAT=.FALSE.               in the INCAR file.

    • 12.08.2003: MAJOR BUG FIX in symmetry.F and paw.F:               for non-collinear calculations the symmetry routines did not work properly

     

     

  • If you have read the previous lines, you will realize that it is recommended to set GGA_COMPAT=.FALSE. for non collinear calculations in VASP.4.6 and VASP.5.2, since this improves the numerical precision of GGA calculations.

 

 

 

 

 

 

 

 

 

 

 

 

 

 



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