[QE-users] Comparison of output from ev.x executable with pressure versus distance obtained from direct numerical derivation

[Lorenzo Paulatto]

Dear Nicolas,
What do you mean by direct numerical derivation? How many points did you
use for the fit, and for the derivation? My guess is that the chisq (chi
squared?) factor is purely accidental.

Regards

-- 
Lorenzo Paulatto CNRS /SU
Written on a virtual keyboard with real fingers

On Sun, 31 Mar 2019, 05:23 Nicolas Leconte, <lecontenicolas0 at uos.ac.kr>
wrote:

> Dear QE users,
>
> I have recently used the ev.x executable to obtain the equation of state
> from my energy versus volume data.
>
> When I compare the Birch Murnaghan (B-M) 3d order equation of state
> results with a direct numerical derivation, I obtain the best matching
> between both methods. Yet, there is still some mismatch.
>
> I tried reducing my data to only include smaller pressures and reduce the
> impact of higher order terms, but it doesn't really change much, while I
> thought it would.
>
> Then, while playing with my output data, I noticed that simply multiplying
> my pressure curve from B-M with the value of chisq, I get perfect agreement
> between both methods.
>
> Am I missing something as on how to use the output from this script. It
> feels like too good a coincidence.
>
> Now, I wonder what precludes me from using this chisq as a prefactor to
> the bulk modulus B0 (
> https://en.wikipedia.org/wiki/Birch%E2%80%93Murnaghan_equation_of_state)
> to obtain a proper agreement with the numerically derived curve.
>
> Regards,
> Nicolas
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