[gpaw-users] Optimizing the unit cell using stress tensor

[sollebac]

Dear gpaw users,

I'm trying to relax a nanotube C  using the stress tensor, then
i set periodic boundary conditions only in z direction  [ pbc=(False,False
True) ]
im wondering if there is a way to fix the  "X" and "Y"  sides of the unit
cell and let  move only in "z" direction?

The nanotube is in a supercell and only is peridic in z ,
here is the script that im using that is similar to example in the web :
 https://wiki.fysik.dtu.dk/gpaw/exercises/stress/stress.html

 i just  want  relax the nanotube  and find the lattice parameter but only
in z direction

from ase import Atoms
from ase.lattice import bulk
from ase.optimize.bfgs import BFGS
from ase.constraints import UnitCellFilter
from gpaw import GPAW
from gpaw import PW
import numpy as np
from ase.structure import nanotube
from ase.io import read,write

a=nanotube(7, 0, length=1, bond=1.4, symbol='C')
a.center()
a.set_pbc((False,False,True))
write('a.traj', a)
calc = GPAW(xc='PBE',
            mode=PW(100),
            # mode=PW(400, cell=a.get_cell()),  # fix no of planewaves!
            kpts=(1, 1, 3),
            # convergence={'eigenstates': 1.e-10},  # converge tightly!
            txt='stress.txt')

a.set_calculator(calc)

uf = UnitCellFilter(a)
relax = BFGS(uf)
relax.run(fmax=0.05)  # Consider much tighter fmax!


b=a.get_cell()[0]
#b=a.get_cell()
print "Unit cell = ",a.get_cell()



any advice is welcome
thank you very much

Jose Luis
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