[Pw_forum] Relaxation of low symmetry lattices

Dal Corso Andrea dalcorso at sissa.it
Thu Oct 8 10:52:52 CEST 2009


On Thu, 2009-10-08 at 10:37 +0200, Stefano Baroni wrote:
> Hi Manoj. the forces you are getting seem really small to me.
> If you enforce cubic symmetry, forces vanish exactly. If you do not,
> the code itself has to "figure out" that the atoms are indeed at their
> proper equilibrium positions, and this can only be achieved within the
> given overall numerical accuracy. One thing that may explicitly
> (although very slightly) break cubic symmetry in your supercell is tha
> FFT grid. Not that it makes any physical sense, but if you really want
> to overkill zero fores, you may want to play a little bit with the
> kinetic-energy cutoff and/or FFT grid. SB

I would also decrease the threshold of the self-consistency conv_thr.

HTH

Andrea 


> 
> On Oct 7, 2009, at 11:23 PM, Manoj Srivastava wrote:
> 
> > Dear PWSCF users, 
> > I have a question regarding relaxation of low symmetry lattices. I
> > did
> > relaxation of 5 atoms FCC unit cell of copper. This is pure copper,
> > no
> > interface, no defect or anything. The unit cell is just not
> > conventional,
> > but a different way of choosing bulk FCC. For the default value of
> > convergence the calculation finishes at the end of 1st scf cycle,
> > but I
> > see different forces when I describe nosym=.TRUE. or cell_parameters
> > {cubic}. Following is the input file.
> > 
> > &control
> >    calculation='relax'
> >    pseudo_dir = '/home/manoj/espresso-4.0.4/pseudo',
> >    outdir='./',
> >    prefix='lcu',
> >    wf_collect=.TRUE.
> > /
> > &system
> >    ibrav =0,
> >    celldm(1)=6.81650937063832
> >    nat= 5,
> >    ntyp= 1,
> >    ecutwfc =50.0,
> >    occupations='smearing',
> >    smearing='gaussian',
> >    degauss=0.02,
> >    ecutrho=500
> > /
> > &electrons
> >    conv_thr = 1.0e-8
> >    mixing_beta=0.7
> > /
> > &ions
> > ion_dynamics='bfgs'
> > pot_extrapolation='second_order'
> > wfc_extrapolation='second_order'
> > /
> > ATOMIC_SPECIES
> > Cu 63.55  Cu.pz-d-rrkjus.UPF
> > ATOMIC_POSITIONS
> >  Cu  0.0   0.0   0.0
> >  Cu  0.5   0.5   0.0
> >  Cu  0.0   1.0   0.0
> >  Cu  1.0   1.0   0.0
> >  Cu  0.5   1.5   0.0
> > 
> > K_POINTS (automatic)
> > 8 8 8 0 0 0
> > CELL_PARAMETERS {cubic}
> > 1.5   0.5  0.0
> > -0.5   1.5  0.0
> > 0.0   0.5  0.5
> > 
> > Result of relaxation : The covergence is achieved with follwing
> > forces on
> > the atoms. 
> > Forces acting on atoms (Ry/au):
> > 
> >   Forces acting on atoms (Ry/au):
> > 
> >     atom   1 type  1   force =     0.00000000    0.00000000
> >    0.00000000
> >     atom   2 type  1   force =     0.00000567    0.00000185
> >   -0.00000172
> >     atom   3 type  1   force =     0.00001155    0.00000377
> >   -0.00000372
> >     atom   4 type  1   force =    -0.00001155   -0.00000377
> >    0.00000372
> >     atom   5 type  1   force =    -0.00000567   -0.00000185
> >    0.00000172
> > 
> > 
> > I realized that I should not have chosen cubic symmetry group, as my
> > unit
> > cell is not cubic. So, as described in the input file parameters,
> > for the
> > low symmetry lattices, I specifed nosym=.TRUE., and again the
> > relaxation
> > stopped after 1 scf cycle, acheiving convergence. But the forces on
> > the
> > atoms are -  
> > 
> > Forces acting on atoms (Ry/au):
> > 
> >     atom   1 type  1   force =     0.00001355    0.00000524
> >   -0.00000516
> >     atom   2 type  1   force =    -0.00001493   -0.00000395
> >    0.00000395
> >     atom   3 type  1   force =     0.00000253   -0.00000043
> >    0.00000045
> >     atom   4 type  1   force =     0.00000820    0.00000230
> >   -0.00000239
> >     atom   5 type  1   force =    -0.00000935   -0.00000316
> >    0.00000315
> > 
> > So, still forces are .001 ev/a.u, which i think is higher for a
> > perfect
> > FCC lattice. I can specify stricter convergence of force, but that
> > would
> > move atoms around and break symmetry. So, as long as the lattice
> > parameter
> > is correct, I think for perfect FCC lattice, forces on atoms should
> > be
> > smaller than what they are presently. I have checked the lattice
> > constant
> > for bulk FCC with a different calculation, so lattice constant is
> > not an
> > issue. Another possibility is symmetry group, as you can see with
> > the same
> > input variables, changing symmetry group changes results.I was
> > wondering
> > if there is a way i can choose correct symmetry group for this low
> > symmetry lattice which might give me smaller forces? Are there any
> > other
> > parameters I need to specify? On a broad view I am curious to know
> > how to
> > do relaxation of low symmetry lattices.
> > 
> > Regards, 
> > Manoj Srivastava
> > University of Florida
> > Gainesville, USA
> > 
> > _______________________________________________
> > Pw_forum mailing list
> > Pw_forum at pwscf.org
> > http://www.democritos.it/mailman/listinfo/pw_forum
> > 
> 
> ---
> Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -
> Trieste
> http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) /
> stefanobaroni (skype)
> 
> 
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> de la pensée - Jean Piaget
> 
> 
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-- 
Andrea Dal Corso                    Tel. 0039-040-3787428
SISSA, Via Beirut 2/4               Fax. 0039-040-3787528
34151 Trieste (Italy)               e-mail: dalcorso at sissa.it





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