[Pw_forum] problem with primitive cell

[Stefano de Gironcoli]

Dear Krishna chaitanya ,
   the problem with eq 2 is that it defines the surface energy as a 
(small) difference between large numbers and tiny inaccuracies in the 
calculation of Ebulk (due to different k-point sampling or limited 
self-consistency) are amplified by the factor proportional to the number 
of layer.
   One way the surfare can be calculated is to rewrite  eq 2 as
   En = n Ebulk + 2 A Ens
   and then extract Ebulk and Ens from a linear fit of calculated En for 
large enough n.
   The slope of the resulting straight line is related to Ebulk and 
should be close to the directly calculated value in a bulk cell while 
the intersection with the vertical axis gives you the surface energy 
(times 2A).
   Hope this helps,
   Stefano de Gironcoli  -  SISSA and DEMOCRITOS

Krishna chaitanya wrote:
> 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-n Ebulk)/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
>  
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>
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