Hi all,
I'm a recent convert to Espresso. I've computed the bond dissociation energy
(BDE) of the CO molecule ( CO --> C + O ) using several available pseudos for C
and O, but do not find good agreement with the experimental value of 256
kcal/mol. Attached, please find a summary of the calculations using the various
pseudos.
Most calculations overpredict the BDE by almost 15 kcal/mol, while the one
using the BLYP pseudo underpredicts it by 11.6 kcal/mol. The BDE obtained using
the pseudos included in example03 and that using the PZ (LDA) pseudo give the
worst result (almost 40 kcal/mol too high). For reference, I have also included
the value obtained by using CASTEP and the PW91 functional, which yields a
value within 5 kcal/mol of the experimental value.
I'd like to clarify that I am using the triplet states for both O and C in
these calculations. I realize that there might be some errors in my input
files, so include a typical input file towards the end of this mail. (the only
change in the other files is the name of the pseudo)
Finally, I'd like to share my ultimate motive behind these calculations - I'm
looking to calculate the binding energies of small molecules on nickel
surfaces. I find that in case of CO, the binding energy is lower by about 15
kcal/mol - the same difference as the BDE - as compared to CASTEP. So my guess
is that the absolute energy of CO molecule is not being calculated correctly
with my current settings.
Any thoughts will be appreciated. Also, I should add that I have browsed the
archive carefully and found no reference to this or similar problems there, but
apologize if I've missed any relevant information (in which case please give a
pointer to that discussion thread).
*********
Input file
*********
&CONTROL
calculation = "relax",
prefix = "CO",
pseudo_dir = "c:/Ameya/Espresso/espresso-3.1.1/pseudo",
outdir = "c:/Ameya/Espresso/espresso-3.1.1/tmp",
/
&SYSTEM
ibrav = 1,
celldm(1) = 20.D0,
nat = 2,
ntyp = 2,
ecutwfc = 29.105D0,
ecutrho = 144.D0,
/
&ELECTRONS
conv_thr = 1.D-7,
mixing_beta = 0.7D0,
/
&IONS
pot_extrapolation = "second_order",
wfc_extrapolation = "second_order",
/
ATOMIC_SPECIES
O 16.00 O.pbe-van_bm.UPF
C 12.01 C.pbe-van_bm.UPF
ATOMIC_POSITIONS {angstrom}
C 0.000 0.0 0.000
O 0.000 0.0 1.100
K_POINTS {Gamma}
*********
End of Input file
*********
Ameya Joshi, Postdoc
----------------------------------------
Chemical Engineering Department
Colorado School of Mines
Ph. (303)-273-3192
----------------------------------------
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