[Pw_forum] benzoic acid structure optimization

[matt reish]

Axel,

Thanks for the advice. I take your point and agree that my results would be
much more meaningful with a better knowledge base. Hopefully I won't have
any more questions until I can ask one worth asking. My initial goal was to
see whether finding Raman phonon intensities on a system this size was
feasible with the resources I have available and I think I've answered that
question.

It's definitely a testament to the software that a spectroscopist with no
plane-wave experience could get the software "working" in just 3 days. I
guess with ease comes black-box users.

Thanks for your help,

Matt

On Thu, May 3, 2012 at 1:01 AM, Axel Kohlmeyer <akohlmey at gmail.com> wrote:

> On Tue, May 1, 2012 at 11:33 PM, matt reish <reishme04 at gmail.com> wrote:
>
> matt,
>
> > Thanks for the reply, seemed to be going way too fast for what I had
> read I
> > that I should expect. Any thoughts on what would be more reasonable
> cutoffs
> > for wavefunction and density? Would it be reasonable to try to attain a
> > useable result on a "cluster" using 8 processors?
>
> it is not the computational resources that you should worry about,
> but the fact that you are trying to use a tool without knowing how
> its fundamental principles work. quantum espresso has great
> tutorials and there are several good textbooks and papers
> to work through. working with plane waves and pseudopotentials
> is not overly difficult, but using them without knowing what you
> are doing, is like waving around a loaded sawn-off shotgun.
>
> simulation software is brutal: it will often run smoothly and
> produce "numbers", even if you feed it complete nonsense.
>
> cheers,
>    axel.
>
> >
> > Cheers,
> >
> > Matt
> >
> >
> > On Wed, May 2, 2012 at 3:02 PM, Axel Kohlmeyer <akohlmey at gmail.com>
> wrote:
> >>
> >> On Tue, May 1, 2012 at 10:53 PM, matt reish <reishme04 at gmail.com>
> wrote:
> >> > Hi PW list,
> >> >
> >> > I am working with Quantum Espresso (v4.3.2.1) in the hopes of
> attaining
> >> > Raman phonon frequencies of a benzoic acid crystal. I attained the
> >> > optimized
> >> > output geometry from a CASTEP calculation in the supplemental material
> >> > of a
> >> > paper. This geometry was used by the authors to calculate accurate IR
> >> > phonon
> >> > frequencies. I used a 'relax' calculation in Quantum Espresso on the
> >> > optimized geometry but this led to large shifts in the atomic
> positions
> >> > in
> >> > the lattice and unreasonable structures for the molecules in the
> lattice
> >> > after finding the optimized structure. My question is does anyone
> have a
> >> > some experience calculating organic crystals in Quantum Espresso that
> >> > could
> >> > give me some tips?
> >>
> >>
> >> matt,
> >>
> >> you are using a *ridiculously* low wavefunction
> >> and density cutoff, that means your basis set
> >> is *far* too small. unphysical results are the
> >> logical consequence.
> >>
> >> axel.
> >>
> >>
> >> >
> >> > Here is my input file minus the atomic posititions:
> >> >
> >> > &CONTROL
> >> >                        title = 'benzacidscftest' ,
> >> >                  calculation = 'relax' ,
> >> >                 restart_mode = 'from_scratch' ,
> >> >                       outdir = '/home/mreish/tmp/' ,
> >> >                   pseudo_dir = '/usr/share/espresso/pseudo/' ,
> >> >                       prefix = 'batest5' ,
> >> >                      disk_io = 'default' ,
> >> >                        nstep = 600 ,
> >> >  /
> >> >  &SYSTEM
> >> >                        ibrav = 12,
> >> >                            A = 5.4996 ,
> >> >                            B = 5.1283 ,
> >> >                            C = 21.950 ,
> >> >                        cosAB = 0 ,
> >> >                        cosAC = -.128276341 ,
> >> >                        cosBC = 0 ,
> >> >                          nat = 60,
> >> >                         ntyp = 3,
> >> >                      ecutwfc = 5 ,
> >> >                      ecutrho = 40 ,
> >> >  /
> >> >  &ELECTRONS
> >> >             electron_maxstep = 200,
> >> >                     conv_thr = 1.0d-11 ,
> >> >  /
> >> >  &IONS
> >> >  /
> >> > ATOMIC_SPECIES
> >> >     C   12.01000  C.pbe-rrkjus.UPF
> >> >     O   16.00000  O.pbe-rrkjus.UPF
> >> >     H    1.00000  H.pbe-rrkjus.UPF
> >> > ATOMIC_POSITIONS angstrom
> >> >
> >> > K_POINTS automatic
> >> >   1 1 1   1 1 1
> >> >
> >> >
> >> > Cheers,
> >> >
> >> > Matthew Reish
> >> > University of Otago
> >> > Dunedin
> >> > New Zealand
> >> >
> >> > _______________________________________________
> >> > Pw_forum mailing list
> >> > Pw_forum at pwscf.org
> >> > http://www.democritos.it/mailman/listinfo/pw_forum
> >> >
> >>
> >>
> >>
> >> --
> >> Dr. Axel Kohlmeyer
> >> akohlmey at gmail.com  http://goo.gl/1wk0
> >>
> >> College of Science and Technology
> >> Temple University, Philadelphia PA, USA.
> >
> >
>
>
>
> --
> Dr. Axel Kohlmeyer
> akohlmey at gmail.com  http://goo.gl/1wk0
>
> College of Science and Technology
> Temple University, Philadelphia PA, USA.
>
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