[Pw_forum] benzoic acid structure optimization

[Giuseppe Mattioli]

Dear Matt
First, you should try to reach yourself convergence with respect to the wfc cutoff, even if the 
30/240 Ry suggested should be fine for C, O ultrasoft pseudopotentials.
I strongly recommend that you use some kind of dispersion correction (semiempirical or ab-initio, 
flick through the PW.x manual). Dispersion forces are quite relevant in organic crystals. As for the 
computational cost, you may only try and see...
HTH
Giuseppe

On Wednesday 02 May 2012 05:33:21 matt reish wrote:
> 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?
> 
> 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.

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   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
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