hybrid DFT for bulk

Axel akoh... at gmail.com
Sun Jul 27 00:23:40 CEST 2008



On Jul 25, 3:38 pm, molecule <san... at gmail.com> wrote:
> 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?

my guess is that in both cases you are getting something
that looks reasonable more or less by chance.

the common wisdom about new features in gaussian (e.g. periodicity,
k-points) is that it takes at least one major release until they
work reasonably well (provided a lot of people are using those
features).

as far as cp2k goes, you should keep in mind that you should not
use a feature that is "under heavy developement" and "incomplete"
unless you are willing _and_ able to dig into the source code and
help with completing the implementation and testing it.

cheers,
    axel.



> 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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