[Pw_forum] equation of state fitting

[Michael Mehl]

Density Functional Theory is not guaranteed to give a bulk modulus that
matches with experiment, unfortunately, though in most case I've tested it
has been within 10%.

In addition to the equation of state approach, you can try computing the
bulk modulus by numerical differentiation, using B(V) = V E''(V).

Just be careful: the derivative is with respect to the volume V, not alat.

You must have minimum energy structure for your crystal structure at that
volume.  This means minimizing the energy with respect to all atomic
positions and all lattice parameters: b/a, c/a, etc., while keeping the
volume fixed.  If your structure is simple enough you may not need to worry
about this.

On Wed, Aug 16, 2017 at 12:36 PM, Sunetra Das <sunetra.das.88 at gmail.com>
wrote:

> Dear QE users,
>
> I would like to know if there are any other equation of state that we can
> use to fit the 'etot vs alat' graph to find out the equilibrium lattice
> constant. I have been following the QE tutorials to learn the basics where
> it has been demonstrated how to fit a etot vs alat graph using the four
> given equations of state. But none of the obtained equilibrium bulk modulus
> values from those equation fitting match the given experimental value, not
> even close.
> How to improve the fitting of the equations or is there any other equation
> I can use to fit the obtained data?
>
> I am new to QE, any suggestion is highly appreciated.
>
> Thank you.
>
> Regards,
> Sunetra Das
> Assistant Professor,
> MMC College, Kolkata, WB,
> India.
>
>
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Michael J. Mehl, Ph.D.

Kinnear Chair in Physics

The United States Naval Academy

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Library of Crystallographic Prototypes <http://aflow.org/CrystalDatabase/>
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