optimisation problem of F compound

[Axel]

On Apr 23, 5:25 pm, Jörg Saßmannshausen
<jorg.sassm... at strath.ac.uk> wrote:
> Dear Fawzi,
>
> thanks for shedding some light into it (which is easy if it is sunny :-) :-)
>
> I am playing around with that problem now for some time. Previously I used
> BLYP (on a larger system but with the same compound in it) and I got a
> imaginary frequency which was again movement of the F atom.
> So I cut down the system and followed Axel's suggestion to use PBE. As I could
> not find a PBE optimised basis set for F, I tried the two I mentioned. With
> both I had the imaginary frequencies.

there are plenty of basis set variations for F in BASIS_MOLOPT.

axel.


>
> I agree with the forces, I think that was what my gutt tried to tell me. I got
> the feeling the cluster will be very busy tonight ;-)
>
> Greetings to Berlin and thanks!
>
> Jörg
>
> On Donnerstag 23 April 2009 Fawzi Mohamed wrote:
>
>
>
> > On 23-apr-09, at 22:35, Jörg Saßmannshausen wrote:
> > > Dear Fawzi,
>
> > > I actually did:
>
> > > cutoff energy
> > > 400  -183.5957955549
> > > 450  -183.5944280905
> > > 500  -183.5937679158
> > > 550  -183.5935371009
> > > 600  -183.5938000945
>
> > > one of the things I learned from Rachel at the workshop :-)
>
> > > For me it looked like that 500 seems to be ok, so I decided to go
> > > for that
> > > value.
> > > However, these values were for the DZVP-ALLELECTRON basis set. I did
> > > not run a
> > > test for the aug-DZVP-GTH, simply assuming as I did not change the
> > > potential
> > > only the basis set the difference would be not that big?
>
> > normally it is a good assumption to assume that the basis set does not
> > influence much the cutoff, but switching from GAPW (allelectron) to
> > GPW does change things, and a lot, gapw does not put the whole density
> > on the grid, so for it the cutoff is not critical.
>
> > > Or was that a wrong
> > > assumption? What cutoff would you recommend?
>
> > I haven't calculated with F, but from the high exponent in the basis
> > (10.7079832847), and also for p functions yo might need more I would
> > try 800.
> > Think that you need accurate forces, on *everything* and if you have
> > just one F looking just at the total energy might be misleading, if
> > you want to be sure do some steps of MD with HF or something like
> > that, and look at the energy conservation...
>
> > > Thanks for the prompt reply!
>
> > you are welcome.
>
> > > All the best from a wet Glasgow!
>
> > and greetings from sunny Berlin :)
>
> > Fawzi
>
> > > Jörg
>
> > > On Donnerstag 23 April 2009 Fawzi Mohamed wrote:
> > >> I would say that the cutoff of 500 might be too small for the F
> > >> element. Have you tested the cutoff needed for it?
>
> > >> Fawzi
>
> > >> On 23-apr-09, at 18:12, Jörg Saßmannshausen wrote:
> > >>> Dear all,
>
> > >>> I am struggling to optimise a molecular structure with a F atom in
> > >>> it.
> > >>> Whereas I manage to get the optimisation converged, the frequency
> > >>> calculation clearly indicates I am not at the local minimum.
> > >>> Replacing
> > >>> the F atom in the molecule with H (and leave everything else the
> > >>> same)
> > >>> results in a converged structure. I got the gutt-feeling that the
> > >>> problem is the basis set / pseudopotential I am using for F. Would
> > >>> somebody mind to give me a helping hand here?
>
> > >>> I have enclosed the following files:
> > >>> - mol file of the frequency calculation (so you see what I mean)
> > >>> - force_eval section
> > >>> - subsys section
> > >>> - fluor-geo.inp
> > >>> - fluor-freq.inp
>
> > >>> I had no problems with the molecule using a different program, so
> > >>> I am
> > >>> quite confident about the structure. Note: it is a cation, I left
> > >>> the
> > >>> anion out for computational reasons.
>
> > >>> Thanks for your help!
>
> > >>> Jörg
>
> --
> *************************************************************
> Jörg Saßmannshausen
> Research Fellow
> University of Strathclyde
> Department of Pure and Applied Chemistry
> 295 Cathedral St.
> Glasgow
> G1 1XL
>
> email: jorg.sassm... at strath.ac.uk
> web:http://sassy.formativ.net
>
> Please avoid sending me Word or PowerPoint attachments.
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