[Pw_forum] calculation of lattice parameters[ i.e. 'a' and 'c/a' ratio] for HCP metals say Mg.

[mohnish pandey]

Dear Biswaranjan!
                             First try to get the experimental (or
theoretical value done by somebody else)  value of the lattice parameter of
your system. If you are using ultrasoft pseudopotential then ecutrho between
8-10 times of ecut will suffice. For fixing 'ecutwfc' you have to do
"vc-relax"  calculation (see doc-input-pw for more detail) at different
'ecutwfc' then see the convergence with respect to ecut. When the difference
between energies for two consecutive ecutwfc is below the minimum limit of
accuracy you want then choose lower value(difference of 5 Ry. between two
consecutive ecut will suffice). The same procedure you have to repeat for
convergence with respect to k-points. You can write a script for doing all
the calculations in one shot.

On Wed, Sep 8, 2010 at 1:24 PM, <bisu123 at platinum.materials.iisc.ernet.in>wrote:

> Dear all,
>             Can someone tell me the correct methodology to start with
> lattice  parameters estimation for HCP metals. I shall tell
> what I did for my earlier simulations involving Magnesium.
>
> First, I fixed the ecut value and then used the same ecut value to fix the
> ecutrho value. Then using the fixed ecut and ecutrho, I varied the
> k-points to obtain the K-points. in my case I obtained [12 12  9]. Then
> for lattice  parameter calculation, [ having fixed ecut, ecutrho, k-points
> of course]  I fixed  'c/a' ratio initially[ c/a= sqrt(8/3)] and varied the
> 'a' across the ideal value of 'a'[ a=3.21 Angstroms =6.06602 at. units]
> and obtained the value for 'a' corresponding for the minimum energy
> configuration. Then using the obtained 'a' in the previous step, I varied
> the 'c/a' ratio across the ideal value to obtain c/a=1.58, corresponding
> to the minimum energy configuration. However this estimate involves
> significant error since the ideal value for c/a( for Mg=sqrt(8/3)=1.624).
> Then I used the value for 'c/a' in the previous step[ i.e 1.58] and varied
> 'a' to find the value of 'a'.
>
> Finally, I obtained a=6.1 a.u=3.23 angstroms, c/a=1.58. The
> psuedopotential I used for Mg is  Mg.pw91-np-van.UPF .
>
> Thanks all,
>
> regards,
> Biswaranjan Dash
> Research Scholar,
> Deformation Mechanisms Modeling Group,
> C/0- Asst Prof S.Karthikeyan
> Indian Institute of Science,Bangalore
> India
>
>
>
>
>
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-- 
Regards,
MOHNISH,
-----------------------------------------------------------------
Mohnish Pandey
Y6927262,5th Year dual degree student,
Department of Chemical Engineering,
IIT KANPUR, UP, INDIA
+919235721300
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