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

[Nicolas Leconte]

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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