hybrid DFT for bulk

molecule san... at gmail.com
Fri Jul 25 21:40:30 CEST 2008


Hi Juerg,

Your are absolutely right. The correct result should be something
slightly below -4 a.u. because the binding energy is small.
Actually, I just did a g03 calculation using
2916 k points, and the result becomes -4.564a.u. which is very
close to the cp2k result, -4.426a.u., with \Gamma only! Is this
surprising?
Is the more reasonable cp2k result due to the error cancellation
between the lack of long-range exact exchange and the \Gamma-only
k-sampling? or some special techniques are applied in cp2k?

regards,
molecule

On Jul 24, 6:08 pm, Juerg Hutter <hut... at pci.uzh.ch> wrote:
> Hi
>
> just a few additional remarks.
> You are aware that your system is pathological? It most likely
> has a degenerate ground state (for non spin polarized) or
> no gap for spin polarized calculations.
> A quick sanity check for the total energy gives
> 8 * -0.5 = -4 a.u.
> Looks like cp2k is less wrong than G03, if something like that
> exists.
>
> Try an infinite chain of He atoms or H2 molecules. There is a
> small chance that you will get reasonable numbers.
>
> regards
>
> Juerg
>
> ----------------------------------------------------------
> Juerg Hutter                   Phone : ++41 44 635 4491
> Physical Chemistry Institute   FAX   : ++41 44 635 6838
> University of Zurich           E-mail: hut... at pci.uzh.ch
> Winterthurerstrasse 190
> CH-8057 Zurich, Switzerland
> ----------------------------------------------------------
>
> On Thu, 24 Jul 2008, molecule wrote:
>
> > Hi, Teodoro
>
> > I don't want to argue about the wording.
> > I just did comparative calculations
> > using cp2k and g03 for an infinite H chain (1A of separation).
> > The settings are the same for both calculations:
> > The unit cell contains 8 H atoms
> > and its dimension is 8A x 8A x 8A.
> > The basis set is 6-31G* and 4 shells are used for the periodicity.
> > xc=b3lyp, k-sampling=\Gamma
>
> > E(g03_PBC)= -27.6442625829 A.U.
> > E(cp2k_PBC)= -4.426086805926252 A.U.
>
> > Any comment on this?
>
> > cheers,
> > molecule
>
> > On Jul 18, 9:28 pm, Teodoro Laino <teodor... at gmail.com> wrote:
> >> Dear molecule,
>
> >> If you consider the gamma point sampling a numerical error.. well..
> >> guess there's not too much one can comment.. but be cautious to do
> >> such claims in a more aggressive audience!
>
> >> Moreover, keep in mind that Juerg also mentioned the 1/r problem for
> >> the exact exchange.. and since your cell is small.. I wonder.. do you
> >> consider
> >> that a numerical problem as well?
> >> Maybe you think that the whole story of long-range interactions is..
> >> a numerical error? Eager to know your point of view..
>
> >> cheers,
> >> Lost_somewhere_with_more_than_6_hours_delay.
>
> >> On 18 Jul 2008, at 23:09, molecule wrote:
>
> >>> However, the behavior looks more like a bug rather than the
> >>> numerical errors caused by \Gamma-only sampling and the
> >>> minimum images.


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