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Dear all
I was trying to calculate the cohesive energy of diamond.
Following are the details of the calculation.
For bulk Carbon
Unit cell with2 carbon atom.
INCAR
ISTART = 0
NWRITE = 0
LREAL= .FALSE.
LWAVE = .TRUE.
LCHARG = .TRUE.
ISPIN=2
ISMEAR =-5
SIGMA = 0.2
AMIX=0.2
BMIX=0.0001
PREC = MEDIUM
NELM = 60
ISIF = 2
IBRION = 1
EDIFFG = -0.01
VOSKOWN=1
KPOINT
Automatic mesh
0
Monkhorst-Pack
4 4 4
0. 0. 0.
POSCAR
C
1.000000
10.000000 0.000000 0.000000
0.000000 11.000000 0.000000
0.000000 0.000000 12.000000
2
Direct
0.500000 0.500000 0.439583
0.500000 0.500000 0.560417
POTCAR
PAW_GGA C 05Jan2001
4.00000000000000000
parameters from PSCTR are:
VRHFIN =C: s2p2
LEXCH = 91
EATOM = 147.4688 eV, 10.8386 Ry
OUTCAR
FREE ENERGIE OF THE ION-ELECTRON SYSTEM (eV)
---------------------------------------------------
free energy TOTEN = -9.219754 eV

energy without entropy= -9.219754 energy(sigma->0) = -9.219754


FORCE on cell =-STRESS in cart. coord. units (eV/reduce length):
Direction X Y Z XY YZ ZX
--------------------------------------------------------------------------------------
Alpha Z 0.24 0.24 0.24
Ewald -28.88 -40.03 110.11 0.00 0.00 0.00
Hartree 77.22 76.15 124.86 0.00 0.00 0.00
E(xc) -26.75 -26.53 -25.74 0.00 0.00 0.00
Local -131.66 -137.23 -317.69 0.00 0.00 0.00
n-local 8.90 4.11 5.22 0.00 0.00 0.00
augment -0.20 0.06 -0.05 0.00 0.00 0.00
Kinetic 100.85 122.89 94.92 0.00 0.00 0.00
-------------------------------------------------------------------------------------
Total -0.27 -0.34 -8.14 0.00 0.00 0.00
in kB -0.33 -0.41 -9.88 0.00 0.00 0.00
external pressure = -3.54 kB Pullay stress = 0.00 kB

For Free carbon atom:
ISTART = 0
NWRITE = 0
LREAL= .FALSE.
LWAVE = .TRUE.
LCHARG = .TRUE.
ISPIN=2
ISMEAR =0
SIGMA = 0.1
AMIX=0.2
BMIX=0.0001
PREC = MEDIUM
NELM = 60
ISIF = 2
IBRION = 1
EDIFFG = -0.01
VOSKOWN=1
POTIM = 0.5
KPOINTS
Automatic mesh
0
Monkhorst-Pack
1 1 1
0. 0. 0.
POSCAR
C
1.000000
10.000000 0.000000 0.000000
0.000000 11.000000 0.000000
0.000000 0.000000 12.000000
1
Direct
0.500000 0.500000 0.500000
POTCAR
PAW_GGA C 05Jan2001
4.00000000000000000
parameters from PSCTR are:
VRHFIN =C: s2p2
LEXCH = 91
EATOM = 147.4688 eV, 10.8386 Ry
OUTCAR

FREE ENERGIE OF THE ION-ELECTRON SYSTEM (eV)
---------------------------------------------------
free energy TOTEN = -1.342886 eV

energy without entropy= -1.265886 energy(sigma->0) = -1.304386


FORCE on cell =-STRESS in cart. coord. units (eV/reduce length):
Direction X Y Z XY YZ ZX
--------------------------------------------------------------------------------------
Alpha Z 0.06 0.06 0.06
Ewald -7.03 -9.88 -12.63 0.00 0.00 0.00
Hartree 33.04 30.63 27.72 0.00 0.00 0.00
E(xc) -12.72 -12.72 -12.72 0.00 0.00 0.00
Local -69.69 -65.30 -58.99 0.00 0.00 0.00
n-local 5.15 4.87 5.08 0.00 0.00 0.00
augment 0.09 0.10 0.09 0.00 0.00 0.00
Kinetic 51.06 52.13 51.33 0.00 0.00 0.00
-------------------------------------------------------------------------------------
Total -0.03 -0.11 -0.05 0.00 0.00 0.00
in kB -0.04 -0.13 -0.06 0.00 0.00 0.00
external pressure = -0.08 kB Pullay stress = 0.00

The value i am getting is

Ecohesive=-9.2197/2 +1.2658
which is much lower than the experomental value of 7.78 ev per atom
Please help me to workout the difference
Musharaf
Thank you in advance

It seems like the POSCAR may not correspond to what you want, in particular for the bulk carbon.

Hi,

Diamond has a FCC structure and it s not a simple C--C.

Cheers

The structure of C-diamond may be something like this:

C-diamond
3.532600000000000
0.5000000000000000 0.5000000000000000 0.0000000000000000
0.0000000000000000 0.5000000000000000 0.5000000000000000
0.5000000000000000 0.0000000000000000 0.5000000000000000
2
Direct
0.0000000000000000 0.0000000000000000 0.0000000000000000
0.2500000000000000 0.2500000000000000 0.2500000000000000


Hope this helps,
Cheers,

When you have used the correct geometry for diamond and when you have checked that your energies for both the bulk and atomic system are well converged with respect to all settings (k-points, cutoff energy, etc), please also understand that there still remains the issue with how well the specific GGA describes the bulk system and the atomic system.

In order to obtain accurate cohesive energies both the bulk system and atom must be described accurately by the specific xc-functional. For instance, GGA-PBE in many cases gives an acceptable description of the atomic energies and together with the good description for the bulk system it can provide reasonable cohesive energies. However, this is very system dependent and the results can be quite off in other cases.