[Pw_forum] Bond dissociation energy of CO (gas-phase)

[Konstantin Kudin]

--- Ameya Joshi <ajoshi at mines.edu> wrote:

 Hi Ameya,

> I'm a recent convert to Espresso.

 Yes, it is about time that this crusade has started to bring in some
fresh converts :-) Now, if only people could get converted away from
using Microsoft formats such as *.xls ...

>...
>...
>...

> 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.

 It seems that you could always compute the energy of CO on the nickel
surface, as well as a few Angstroms away from the surface. Then you get
the energy difference from an identical setup, just different
geometries. See if you still get the 15 kcal/mol difference. Now, if
you computing the energy of Ni-CO with respect to (Ni+C+O), that one
can easily find this 15 kcal error coming from the improperly handled
reference substances.

 Anyway, BDEs are often rather challenging to compute correctly (one
has to deal with reference atomic states and such), so try to avoid
doing that if possible, and stick to the smallest possible changes.

 And, if you really want to learn about minimizing errors for chemical
reactions, check out the definition of "isodesmic" :
http://en.wikipedia.org/wiki/Isodesmic_reaction

 Kostya

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