[Pw_forum] problem with primitive cell

[Krishna chaitanya]

Hi,

I am trying to calculate surface energy of MgO surface layers using PWSCF.
For that i have used 2 methods given as

1)Ens={En-n(En-En-1)}/2A
    here n=no of layers, En and En-1 are total energies of n and n-1 layers,
A=surface area of primitive cell

2)Ens=(En-mEbulk)/2A
   here m=no of primitive units present in surface, Ebulk=total energy of
primitive unit cell

I have got converged surfae energy 0.004au/bohr**2 by using method 1, but by
using method 2 i got very different energies and not converged by varying
number of layers. When i checked with xcrysden the periodic repetetion in
primitive cell is not good. Initially i have followed by cartesian
coordinates but the number of symmetry operations observed as 1, but when
followed by fractional coordinates symmetry operations are 48 now.


  MgO conventional cell is in FCC bravis lattice group and i have used the
following input for the primitive cell description

&SYSTEM

ibrav = 2,

A = 2.977768,

B = 2.977768,

C = 2.977768,

cosAB = 0.5,

cosAC = 0.5,

cosBC = 0.5,

nat = 2,

ntyp = 2,

ecutwfc = 25,

ecutrho = 200,

/

&ELECTRONS

electron_maxstep = 500,

diago_david_ndim = 6,

/

&IONS

ion_dynamics = 'bfgs',

/

ATOMIC_SPECIES

O 16.000 O.pw91-van_ak.UPF

Mg 24.305 Mg.pw91-np-van.UPF

ATOMIC_POSITIONS crystal

Mg 0.00000000 0.00000000 0.00000000

O 0.50000000 0.50000000 0.50000000

K_POINTS automatic

4 4 4 0 0 0


Please suggest me where i have done mistake and thanks in advance

G. Krishna Chaitanya
Senior research fellow
C/o Dr. Bhanuprakash K.
Inorganic Chemistry Division
Indian Institute of Chemical Technology
Hyderabad, India
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.democritos.it/pipermail/pw_forum/attachments/20081118/fbfe00e4/attachment-0001.htm