[Pw_forum] benzoic acid structure optimization

[Axel Kohlmeyer]

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.