二维材料的HSE+SOC的计算

这里主要讨论一下用vasp对二维材料的hse+soc对能带剪刀操作的过程。

1. 结构优化

我们对材料Ag2Br2进行结构优化，结构与优化的参数如下：

image.png

进行了分步优化，下面为最后一步的优化参数

SYSTEM = stepOpt6
NPAR = 4
ALGO = Normal  # Fast;Normal
LREAL = .FALSE.
#LPLANE = .TRUE.
PREC = Accurate
#AMIN = 0.01
#ISYM = 0

##2_INIT_input
ISTART = 0
ICHARG  = 2

##3_ELECTRON
NELM = 200
NELMIN = 5
EDIFF = 1e-6
ISMEAR = 0
SIGMA = 0.05
#LMAXMIX = 4
ENCUT = 600
ISPIN =1 

##4_IONS
NSW = 200
IBRION = 2
POTIM = 0.5
ISIF = 3
EDIFFG = -0.001
ADDGRID = .TRUE.

##5_Kmesh
KSPACING = 0.189

##6_Output
LWAVE = .False.
LCHARG = .False.
LORBIT = 11

####optB86-vdw####
GGA = MK
PARAM1 = 0.1234
PARAM2 = 1.0000
LUSE_VDW = .True.
AGGAC = 0.0000

2.自洽(即输出波函数和电荷密度)

这里考虑到后续的真空层对齐，因而打开了静电势的开关LVHAR(此tag的详细的问题可以参照这个链接)

##1_system
SYSTEM = stepOpt6
NPAR = 4
ALGO = Normal  # Fast;Normal
LREAL = .FALSE.
#LPLANE = .TRUE.
PREC = Accurate
#AMIN = 0.01
#ISYM = 0

##2_INIT_input
ISTART = 0
ICHARG  = 2

##3_ELECTRON
NELM = 200
NELMIN = 5
EDIFF = 1e-6
ISMEAR = 0
SIGMA = 0.05
#LMAXMIX = 4
ENCUT = 600
#ISPIN =1 

##4_IONS
NSW = 0
IBRION = -1
POTIM = 0.5
ISIF = 2
EDIFFG = -0.001
ADDGRID = .TRUE.

##5_Kmesh
#KSPACING = 0.189

##6_Output
LWAVE = .True.
LCHARG = .True.
LORBIT = 11

####optB86-vdw####
GGA = MK
PARAM1 = 0.1234
PARAM2 = 1.0000
LUSE_VDW = .True.
AGGAC = 0.0000

#### Add Parameters ####
LVHAR = .True.

3. pbe的band计算

band计算的时候根据高对称点自己线性插值生成高对称路径k点

SYSTEM = stepOpt6
NPAR = 4
ALGO = Normal  # Fast;Normal
LREAL = .FALSE.
#LPLANE = .TRUE.
PREC = Accurate
#AMIN = 0.01
#ISYM = 0

##2_INIT_input
ISTART = 1
ICHARG = 11

##3_ELECTRON
NELM = 200
NELMIN = 5
EDIFF = 1e-6
ISMEAR = 0
SIGMA = 0.05
#LMAXMIX = 4
ENCUT = 600
ISPIN =1 

##4_IONS
NSW = 0
IBRION = -1
POTIM = 0.5
ISIF = 2
EDIFFG = -0.001
ADDGRID = .TRUE.

##5_Kmesh
#KSPACING = 0.189

##6_Output
LWAVE = .False.
LCHARG = .False.
LORBIT = 11

####optB86-vdw####
GGA = MK
PARAM1 = 0.1234
PARAM2 = 1.0000
LUSE_VDW = .True.
AGGAC = 0.0000

4. HSE+SOC(进入正题)

由于PBE存在低估带隙的问题，因而需要考虑仅HSE的计算，另外在批量计算的时候，我们可能要引入SOC来保证计算的严谨性。
但需要注意的是，即便对于我所计算的二维材料(仅有4个原子)，当仍然面对该类计算相当耗费算力。不废话，直接说一些测试下来，必然对的点：

k点会显著影响计算速度，大致呈现的时间效率;
相同k点数下，读取用PBE得到的WAVECAR将节省~60-70%的计算时间(相较自己撒点);
ENCUT对于计算速度也有较大影响需要进行测试
此类计算只需要读取WAVECAR(ISTART = 1)不需要读取(ICHARGE = 2)

首先需要做一步带SOC的自洽计算:

SYSTEM = stepOpt6
NPAR = 4
ALGO = Normal  # Fast;Normal
LREAL = .False.
LPLANE = .False.
PREC = Accurate
#AMIN = 0.01
#ISYM = -1

##2_INIT_input
ISTART = 0
ICHARG  = 12

##3_ELECTRON
NELM = 200
NELMIN = 5
EDIFF = 1e-6
ISMEAR = 0
SIGMA = 0.05
#LMAXMIX = 4
ENCUT = 325
#ISPIN =1 

##4_IONS
NSW = 0
IBRION = -1
POTIM = 0.5
ISIF = 2
EDIFFG = -0.001
ADDGRID = .TRUE.

##5_Kmesh
KSPACING = 0.4

##6_Output
LWAVE = .True.
LCHARG = .True.
LORBIT = 11

####optB86-vdw####
#GGA = MK
#PARAM1 = 0.1234
#PARAM2 = 1.0000
#LUSE_VDW = .True.
#AGGAC = 0.0000

#### Add Parameters ####
#LVHAR = .True.

#### mbj ####
#METAGGA = MBJ
#LASPH = .TRUE.
#LMIXTAU = .TRUE.

#### mix wave####
#IMIX = 1
#AMIX = 0.2
#BMIX = 0.0000001

#### HSE ####
#LHFCALC = .TRUE.
#HFSCREEN = 0.2

#### SOC ####
LSORBIT = .TRUE.
SAXIS = 0 0 1
MAGMOM = 1000*0
GGA_COMPAT = .FALSE.
#---for slabs---#
#AMIX = 0.2
#BMIX = 0.00001
#LSCALAPACK = .FALSE.

随后做能带剪刀操作:

将IBZKPT文件负值为KPOINTS同时加入PBE计算的带边K点(不要忘了改一下k点总数[+2])

下面给出测试结果的比较：

测试材料：Ag2Br2 (4原子)	ISTART = 1	ISTART = 1	ISTART = 0(自己撒点)	ISTART = 0(自己撒点)
ENCUT	325	325	325	600
KSPACING	0.45	0.4	自己撒点()	自己撒点()
K点数(不算带边k点)	16	25	13	13
gap	2.8935	2.8913	2.9043	2.9096
用时	3835.938	10405.274	8078.797	49828.280

因而可以发现测试下来只需要用KSPACING = 0.45做+SOC的自洽计算同时选取ENCUT = ENMAX*1.3就能保证计算。

另外，需要注意的是在ISTART = 1 的计算中都出现了以下的问题，一开始尝试解决，但发现后来最后能算出来。在不读WAVECAR(ISTART = 0)的计算中不会出现，可能与WAVEACR的读取有关。