[Pw_forum] problem with primitive cell

Krishna chaitanya krishnachaitanya.gunturu at gmail.com
Tue Nov 18 05:16:28 CET 2008


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

    here n=no of layers, En and En-1 are total energies of n and n-1 layers,
A=surface area of primitive cell

   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


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,



electron_maxstep = 500,

diago_david_ndim = 6,



ion_dynamics = 'bfgs',



O 16.000 O.pw91-van_ak.UPF

Mg 24.305 Mg.pw91-np-van.UPF


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
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