[gpaw-users] Inplane force != zero

[Duy Le]

Dear All,
I was relaxing 5 layer of Cu(111). For (1x1x5) slab, everything is great.
However, for (5x5x5) slab, It is weird that the in-plane components of
forces (in Oxy plane) are not zero (they should be zero because
of symmetry).
I don't know what causes the problem (or maybe not).
Any comment will be great!


******Input file:
#!/usr/bin/env python
from ase import *
from ase.lattice.surface import *
from gpaw import GPAW, Mixer, FermiDirac
from gpaw.utilities.tools import cutoff2gridspacing, gridspacing2cutoff
a = 3.676
h=a*sqrt(1./3.)  # layer distance
d=a/sqrt(2.)     # bond lengths
vac=h*5.
system0 = read('1x1x5.xsf')
system = system0*(5,5,1)
system.set_pbc((1,1,1))
output='5x5x5.txt'
xsffile='5x5x5.xsf'
tmpfile='5x5x5.gpw'
traj='5x5x5.traj'
k=1
ngx=64
ngy=64
ngz=148
fa = FixAtoms(mask=[a.position[2] == system.cell[2,2]/2. for a in system])
system.set_constraint([fa])

calc=GPAW(kpts=(k,k,1),occupations=FermiDirac(0.1),
          xc='revPBE',
          gpts=(ngx,ngy,ngz),
          convergence={'energy':0.0001,
                       'density':1.e-4,
                       'band':'occupied'},
          txt=output)
system.set_calculator(calc)
Etot1 = system.get_potential_energy()

dyn = BFGS(system,trajectory=traj)
dyn.run(fmax=0.01)
calc.write(tmpfile)
write(xsffile,system)

--------------------------------------------------
Duy Le
PhD Student
Department of Physics
University of Central Florida.

"Men don't need hand to do things"
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