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" <[email protected]> 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 > > __________________________________________________ > Do You Yahoo!? > Tired of spam? Yahoo! Mail has the best spam protection around > http://mail.yahoo.com > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum
