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

[Axel Kohlmeyer]

On 9/6/06, Ameya Joshi <ajoshi at mines.edu> wrote:
> 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

i agree for the ultra-soft potentials (and even there are differences, as one
can kindof define during psp generation, how soft the potential should be)
but for the norm-conserving ones used with BLYP, you'll need 70-90ry.
and in general you always have to find the optimal cutoff for each pair
as the 'hardest' pseudopotential determines the required cutoff.

cheers,
   axel.
> 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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-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
  Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.