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

[Paolo Giannozzi]

On Sun, Mar 31, 2019 at 4:34 PM Lorenzo Paulatto <paulatz at gmail.com> wrote:

My guess is that the chisq (chi squared?)
>

yes, chi squared, as usually defined: (1/N) \sum_{i=1}^N (E_i^{fit}-E_i)^2

factor is purely accidental.
>

I cannot but agree

Paolo

>
> 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
>> _______________________________________________
>> users mailing list
>> users at lists.quantum-espresso.org
>> https://lists.quantum-espresso.org/mailman/listinfo/users
>
> _______________________________________________
> users mailing list
> users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users



-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20190331/2e4235b1/attachment.html>