CrI3 bulk 结构优化

最近看了一篇文章，是关于铬的卤化物 CrX3 的诸多性质的计算

Robust intrinsic ferromagnetism and half semiconductivity in stable two-dimensional single-layer chromium trihalides
Wei-Bing Zhang et al.

我尝试着重复这篇文章里的计算。因为我对整个计算流程其实不太熟悉，所以在此详细记录一下整个过程，作为学习笔记。

INCAR

     SYSTEM = CrI3 bulk
       ISIF = 3
        NSW = 1000
      EDIFF = 1E-5
     EDIFFG = -5E-3
       PREC = High
     IBRION = 2
     ISMEAR = 0
      ISPIN = 2
      ENCUT = 600
#vdw
       #GGA = RE
  #LUSE_VDW = .TRUE.
     #AGGAC = 0.0000

各个参数的意义，待说明

POSCAR

具体参考上一篇博客如何得到 CrI3 bulk 的 POSCAR

CrI3 bulk system
1.0
        7.7013001442         0.0000000000         0.0000000000
        4.6397266689         6.1467845536         0.0000000000
        4.6397266689         2.3109422460         5.6958323610
    I   Cr
    6    2
Direct
     0.419150014         0.070070001         0.767089986
     0.580850148         0.929929961         0.232909974
     0.767089920         0.419150013         0.070069996
     0.232909981         0.580850061         0.929929980
     0.070069968         0.767090064         0.419149980
     0.929930017         0.232909969         0.580850001
     0.167169998         0.167170023         0.167169997
     0.832829889         0.832830068         0.832829984

KPOINTS

k-points
 0
Monkhorst Pack
 8  8  8
 0  0  0

kpoints 的选取是按照论文中的取法

K -points sampling with a mesh of 8×8×8 kpoints generated by the scheme of Monkhorst-Pack is used for a primitive cell of bulk CrX3 as shown in Fig. 1- (b).

POTCAR

论文里提到用赝势用 PAW_PBE 的方法

The present calculations have been performed with spin-polarization using the Vienna ab initio simulation package (VASP) code20,21 within projector augmentedwave (PAW) method22,23. General gradient approximations (GGA) in the Perdew-Burke-Ernzerhof(PBE) implementation24 are applied as the exchange correlation functional.

因此，先找到 PAW_PBE 的目录，因此导入 I 和 Cr 的POTCAR，再将 POTCAR 转移到计算目录下即可

$ zcat /share/apps/src/vasp/pseudopotential/paw_pbe/I/POTCAR.Z > POTCAR
$ zcat /share/apps/src/vasp/pseudopotential/paw_pbe/Cr/POTCAR.Z >> POTCAR

计算结果 CONTCAR

计算完成得到了 CONTCAR，与上一篇博客得到POSCAR 的操作类似，把 CONTCAR 导入 VESTA 得到 cif，再导入 Materials studio，通过 Find Symmetry 可以得到结构优化以后的晶格参数 a 和 c，如下图

对比论文中给出的实验数据

在开始计算时，我输入POSCAR的是 rhombohedral structure, a = 6.867 A，c = 19.807 A，第一步结构优化得到的 CONTCAR 显示优化后 a = 7.0038 A, c = 22.4418。与论文中 PBE 计算的 CrI3 的结果 7.006 和 22.35 可以说是基本一致了。