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Hi, everbody

I calculated the isolated NO molecule, the total energy Etot=-12.026eV. But the experimental data of bond energy N=O is 607KJ/mol, is about 6.294eV. I do not know why, so I ask for your help. My setup is as follow:

INCAR
general:
SYSTEM = NO
ISTART = 0 ; ICHARG=2
ISMEAR = 0 ; SIGMA = 0.2
dynamic:
IBRION = 2
NSW = 100
POTIM = 0.1
EDIFFG=-0.01
ENCUT=400
PREC = High
################################
POSCAR
1.00000000000000
30.8200000000000003 0.0000000000000000 0.0000000000000000
0.0000000000000000 30.8200000000000003 0.0000000000000000
0.0000000000000000 0.0000000000000000 30.3150000000000013
O N
1 1
Selective dynamics
Direct
0.7378683101883881 0.9468784104989977 0.9995057216026794 T T T
0.7352416702863921 0.9874461343450991 0.0000494644823208 T T T
##########################
KPOINT
Gamma-point only
1
rec
0 0 0 1
##########################
POTCAR use normal PBE

##########################

Thank you for your help and look forward to your answer
Best wishes

Hi!

The energy of formation is NOT equal to the total energy.

I'd strongly(!) suggest you read some basic textbooks about thermodynamics and quantum chemistry/physics.

Cheers,

alex

I also considered spin polarization. with ISPIN=2 Etot_NO=-12.2979eV,and E_O=-1.5925eV E_N=-3.1112eV, Ebond=7.5942eV, which is better a little bit, but still 1.3eV larger than the experimental value.

alex wrote:Hi!

The energy of formation is NOT equal to the total energy.

I'd strongly(!) suggest you read some basic textbooks about thermodynamics and quantum chemistry/physics.

Cheers,

alex

hi alex

thanks for your answer

sure, you are right, "The energy of formation is NOT equal to the total energy." But if you calculate an isolated atom like O N in a normal way(without spin polarization), the energy is rather small comparing with the total energy of molecule, that's why I only mentioned the total energy (I am sorry that I forget to give the results with spin polarization, and I have added this part already). I think "formation energy" is not what I am talking about,but "binding energy". For formation energy, one must think about the reference.
Thank you again for your answer

Best wishes

Hi tonylxh,

I see your point. Anyway, scientific discussion needs precision in both language and experiment. ;-)
Thanks for clarification. Try something like that for INCAR (see below).

One more thing - you have done right in your POSCAR already - is to destroy symmetry in one direction.
You also need to check you OUTCAR for the proper occupation.

Cheers,

alex

PS:

SYSTEM = O-atom

Electronic minimisation
GGA = PE
PREC = Accu
EDIFFG = 1.e-8
EDIFFG = -1.e-6
ENCUT = 400.
NELMIN = 4
NELM = 200 ! Maximum number of electronic SC steps
LMAXMIX = 6
ISPIN = 2
NUPDOWN = 2

DOS related values
SIGMA = 0.001
ISMEAR = 0 ! Gaussian smearing for big cells

Do not write huge files
LCHARG = .FALSE.
LWAVE = .FALSE.

Performance options
NPAR = 1 ! adjust to number of processors
ALGO = N

alex wrote:Hi tonylxh,

I see your point. Anyway, scientific discussion needs precision in both language and experiment. ;-)
Thanks for clarification. Try something like that for INCAR (see below).

One more thing - you have done right in your POSCAR already - is to destroy symmetry in one direction.
You also need to check you OUTCAR for the proper occupation.

Cheers,

alex

PS:

SYSTEM = O-atom

Electronic minimisation
GGA = PE
PREC = Accu
EDIFFG = 1.e-8
EDIFFG = -1.e-6
ENCUT = 400.
NELMIN = 4
NELM = 200 ! Maximum number of electronic SC steps
LMAXMIX = 6
ISPIN = 2
NUPDOWN = 2

DOS related values
SIGMA = 0.001
ISMEAR = 0 ! Gaussian smearing for big cells

Do not write huge files
LCHARG = .FALSE.
LWAVE = .FALSE.

Performance options
NPAR = 1 ! adjust to number of processors
ALGO = N

Hi Alex

Thank you very much for your answer. I tried to calculate the energy of O atom with INCAR you suggusted, but the energy does not change much(less than 0.05eV). I am wondering if there is any mistake I made when I calculated the total energy of NO. Do you find any mistake of my input files?
Thanks again for your help

Best wishes
tony

Hi tony,

you got the wrong occupation in your calculation. If you do it right, you'll end up at around -1.9 eV for the total energy of an O atom (400eV/O standard PAW/PBE).

cheers,

alex

alex wrote:Hi tony,

you got the wrong occupation in your calculation. If you do it right, you'll end up at around -1.9 eV for the total energy of an O atom (400eV/O standard PAW/PBE).

cheers,

alex

Hi alex

Thank you very much for your help

Best wishes
tony