[Pw_forum] Calculation of lattic constants and bulk modulus od w-ZnO

[Nicola Marzari]

Dear Zavar,


both your approach and the one suggested by Mohnish are good -
vc_relax is more straightforward, but the one you have chosen
requires you to think a bit more about the problem (and it's good
for that). While you are learning, try both, and see if they give you
the same results.

As a side notice, you typically need a larger cutoffo to converge well
the stress tensor, used by vc_relax - hence your approach of minimizing 
the energy can be more accurate, at usual cutoffs.

			nicola



On 13/05/2012 08:35, mohnish pandey wrote:
> Dear Muhammad,
>
> Their is an easier way to calculate the optimized the lattice constant
> which is based on calculation of stress and forces on the system. In the
> flag "calculation" put "vc-relax" and in "ion_dynamics" put "bfgs"
> (there are other options too) and in the "cell_dynamics" put "bfgs"
> (again there are other options also) and run the job. When the
> calculation is over the code will print the final lattice vectors by
> which you can calculate the optimized lattice constant. For further
> details look into /quantum-espresso-folder/Doc/INPUT_PW.html
>
>
> On Sun, May 13, 2012 at 2:52 AM, zafar rasheed <zafartariq2003 at yahoo.com
> <mailto:zafartariq2003 at yahoo.com>> wrote:
>
>     Dear All
>
>     I want to calculate lattice constants a, and c and bulk modulus of
>     w-ZnO. For this purpose:
>
>      1.
>
>         I take experimental lattice constants (a= 3.25 Ang. and c =
>         5.206 Ang. and by changing these values into bohr.) and first
>         take different values above and below a and do scf calculations
>         to optimize a. As in case of W ZnO (Hexagonal and Trigonal P)
>         ibrav= 4 and celldm(3) =c/a is used. So when I change a (celldm
>         (1))I have to change celldm (3). In this way I get lattice
>         constant a which is nearly same as experimental with 2 percent
>         error.
>
>      2.
>
>         Then I fix this lattice constant a (celldm (1)) and change
>         lattice constant c (By taking different values above and below
>         experimental value of c). As c changes celldm (3) also changes.
>         In this way I get lattice constant c which is very close to
>         experimental lattice constant with a little error of 2 percent.
>
>      3.
>
>         The output data for the calculation of c by fixing a I use to
>         calculate BULK modulus for ZnO. Again these results are
>         comparable with experimental and already calculated results by
>         different techniques .
>
>      4.
>
>         Now I want to know that, Is my calculation procedure is right?
>
>     Best Regards
>
>     Muhammad Zafar
>     PhD Scholar
>     Department of Physics
>     The Islamia University of Bahawalpur,Pakistan
>
>

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Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL