H 的结合能

首先要计算一个氢气分子的孤立能量，再减去两个孤立氢原子能量，将得到氢气分子的结合能。

1. 先计算单个原子能量，POTCAR选取PAW_GGA

INCAR:

PREC=Accurate
ISMEAR=0; SIGMA=0.05
ISPIN=2

KPOINTS:

Monkhorst Pack
0
Monkhorst Pack
1 1 1
0 0 0

POSCAR:

H atom
1
15.0  0.0   0.0
0.0   15.0  0.0
0.0   0.0  15.0
1
Cart
0 0 0

得到的能量如下：

Free energy of the ion-electron system (eV) 
--------------------------------------------------- 
alpha Z        PSCENC =         0.00072437 
Ewald energy   TEWEN  =        -1.36188267 
-1/2 Hartree   DENC   =        -6.92165960 
-V(xc)+E(xc)   XCENC  =         2.34398347 
PAW double counting   =         0.74203130       -0.75105109 
entropy T*S    EENTRO =         0.00000000 
eigenvalues    EBANDS =        -7.60619631 
atomic energy  EATOM  =        12.44420649 
--------------------------------------------------- 
free energy    TOTEN  =        -1.10984404 eV

energy without entropy =       -1.10984404  energy(sigma->0) =       -1.10984404

2， 计算H2分子的能量

INCAR:

PREC=Accurate
ISMEAR=0; SIGMA=0.05
ISPIN=2
NSW=5
IBRION=2
POTIM=0.1

KPOINTS与计算H原子能量时相同

POSCAR:

H2 molecular
1
8.0  0.0   0.0
0.0   8.0  0.0
0.0   0.0  8.0
2
Cart
0 0 0
0 0 0.5

得到的能量如下：

Free energy of the ion-electron system (eV) 
--------------------------------------------------- 
alpha Z        PSCENC =         0.01909948 
Ewald energy   TEWEN  =         9.03591211
-1/2 Hartree   DENC   =       -25.32819184 
-V(xc)+E(xc)   XCENC  =         5.31187543 
PAW double counting   =         8.66129443       -8.70651972 
entropy T*S    EENTRO =         0.00000000 
eigenvalues    EBANDS =       -20.61147888 
atomic energy  EATOM  =        24.88841298 
--------------------------------------------------- 
free energy    TOTEN  =        -6.72959600 eV

energy without entropy =       -6.72959600  energy(sigma->0) =       -6.72959600

得到的CONTCAR文件如下：

H2 molecular                                     
1.00000000000000     
8.0000000000000000    0.0000000000000000    0.0000000000000000 
0.0000000000000000    8.0000000000000000    0.0000000000000000 
0.0000000000000000    0.0000000000000000    8.0000000000000000 
2
Direct 
0.0000000000000000  0.0000000000000000 -0.0155839976608132 
0.0000000000000000  0.0000000000000000  0.0780839976608132 
  
0.00000000E+00  0.00000000E+00  0.00000000E+00 
0.00000000E+00  0.00000000E+00  0.00000000E+00


3，计算得到结合能E=-(TOTEN(H2)-2*TOTEN(H)) =-（-6.72959600-2*-1.10984404）= 4.51eV，H-H键长r=sqrt{(x2-x1)^2+(y2-y1)^2+(z2-z1)^2}=(0.0780839976608132+0.0155839976608132)*8=0.74934 埃，