Atomization energy of small molecules

Message

Joan Adler · #1 Post by **Joan Adler** » Wed Jul 20, 2011 2:28 pm

As part of my calculation I am trying to calculate atomization energy for
small molecules (H2, O2, CO, CO2)
I am using ase simulation environment with vasp
example for H2:
the calculation is done as:

E_atomization=E_molecule-2*Eatom

I found this discussion on the support site
http://cms.mpi.univie.ac.at/vasp-forum/ ... php?4.3340

there it seems that yzt102 had the same problem. I tried the suggestions
given there but it didn't seem to help.
I am getting energy of 0.9eV
I expected to get values of ~1.1 for free hydrogen atom

my INAR file is:
INCAR created by Atomic Simulation Environment
SIGMA = 0.001000
EDIFF = 1.00e-07
PREC = High
ISPIN = 2
NBANDS = 4
ISMEAR = 0
NSW = 0
NELM = 60
LREAL = .FALSE.

KPOINTS created by Atomic Simulation Environemnt
0
Monkhorst-Pack
1 1 1
0 0 0

POSCAR

H
1.0000000000000000
8.0000000000000000 0.0000000000000000 0.0000000000000000
0.0000000000000000 8.0000000000000000 0.0000000000000000
0.0000000000000000 0.0000000000000000 8.0000000000000000
1
Cartesian
4.0000000000000000 4.0000000000000000 4.0000000000000000

POTCAR parameters:

PAW H 06May1998
1.00000000000000000
parameters from PSCTR are:
VRHFIN =H: ultrasoft test
LEXCH = CA
EATOM = 12.1334 eV, .8918 Ry

TITEL = PAW H 06May1998
LULTRA = F use ultrasoft PP ?
IUNSCR = 0 unscreen: 0-lin 1-nonlin 2-no
RPACOR = .000 partial core radius
POMASS = 1.000; ZVAL = 1.000 mass and valenz
RCORE = 1.100 outmost cutoff radius
RWIGS = .700; RWIGS = .370 wigner-seitz radius (au A)
ENMAX = 250.000; ENMIN = 200.000 eV
RCLOC = .701 cutoff for local pot
LCOR = T correct aug charges
LPAW = T paw PP
EAUG = 400.000
RMAX = 2.177 core radius for proj-oper
RAUG = 1.200 factor for augmentation sphere
RDEP = 1.112 core radius for depl-charge
QCUT = -5.742; QGAM = 11.483 optimization parameters

I am running on Ubuntu 10.04.1

Can anyone Help me here?
Thanks
Joan Adler

kambiz · #2 Post by **kambiz** » Thu Jul 21, 2011 10:49 am

Hi,

I ve done this before, and I got -1.11 eV.

This is my INCAR:

SYSTEM = H
PREC = Medium
ENCUT = 550
ISMEAR = 0
SIGMA = 0.001
NSW = 0
EDIFF = 0.0001
GGA = PE
NBANDS = 45
LPLANE = .FALSE.
LREAL = .FALSE.
ISPIN = 2
ICHARG = 2
ISTART = 0
ISYM = 0

I think you should switch off the symmetry.

And I also used Gamma-point only in my KPOINTS file.

Cheers
Kambiz

jber · #3 Post by **jber** » Thu Jul 21, 2011 7:15 pm

Calculation of atomization energies depending on the precision you need may be tricky,
because the default (ALGO = Normal) SCF seems to be unstable with respect to the number of bands for atoms, see
http://cms.mpi.univie.ac.at/vasp-forum/ ... php?4.9673
The error of the total energy from changing the number of bands can reach 0.005 eV per atom,
so you will get into troubles when calculating larger molecules.
You should be fine for small systems (you can get away with the default vasps' number of bands).
Check your results with respect to:
- hard potential (H_h)
- default number of bands (don't set NBANDS)
- SIGMA = 0.0
- cell with a broken symmetry
In case you need N or P elements, you will have to set occupations by hand.