[Pw_forum] Relaxation of low symmetry lattices

[Manoj Srivastava]

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