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

[Ameya Joshi]

Axel and Kostya,

Thanks for the suggestions. My first concern was convergence, and as pointed 
out by Axel, I did try increasing the cutoff, but even after increasing the 
cutoff from 30 to 50 Ryd, the BDE changes only by 0.5 kcal/mol (and makes it 
worse), so its safe to say its converged with respect to that. I've also 
confirmed that increasing the box size has no further effect.

Ameya




----- Original Message ----- 
From: "Konstantin Kudin" <konstantin_kudin at yahoo.com>
To: <pw_forum at pwscf.org>
Sent: Wednesday, September 06, 2006 4:20 PM
Subject: Re: [Pw_forum] Bond dissociation energy of CO (gas-phase)


> --- 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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