[Pw_forum] Relaxation of low symmetry lattices

[Stefano Baroni]

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

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
>
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---
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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