<div dir="ltr"><div>Hi Lorenzo and Paolo,</div><div><br></div><div>Thanks for your responses.</div><div><br></div>Numerical derivation in the sense of deriving the energy with distance to get the force, and derive by the surface to get the pressure.<div><br></div><div>For both fit and derivation, between 3.0 and 3.6 Angstrom, I used 1000 points (through interpolation of the DFT data). I further tested it with 1500 points (where I had to change the hard-coded limit for the size of one of the vectors in ev.f90). It did not change the behavior much.</div><div><br></div><div>I would also imagine it to be accidental, but I was surprised to see such a good agreement. So I was vaguely hoping for some misunderstanding from my part or even a bug in the code I was not aware of.</div><div><br></div><div>Option 2, among the 4 options, Birch-Murnaghan was giving best agreement, which seems to make sense due to the higher order terms. But maybe by reducing the range of compression I could get a better agreement. Right now, compression from 3.4 to 3.0 Ang of my bilayer interlayer distance requires about 10 GPa. </div><div><br></div><div>Maybe that's too much to hope for a good equation of state fitting?</div><div><br></div><div>Regards,</div><div>Nicolas</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Apr 1, 2019 at 4:19 AM Paolo Giannozzi <<a href="mailto:p.giannozzi@gmail.com">p.giannozzi@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div dir="ltr">On Sun, Mar 31, 2019 at 4:34 PM Lorenzo Paulatto <<a href="mailto:paulatz@gmail.com" target="_blank">paulatz@gmail.com</a>> wrote:</div><div dir="ltr"><br></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="auto">My guess is that the chisq (chi squared?)</div></blockquote><div> </div><div>yes, chi squared, as usually defined: (1/N) \sum_{i=1}^N (E_i^{fit}-E_i)^2</div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="auto"><div dir="auto"><div dir="auto"><div dir="auto">factor is purely accidental.</div></div></div></div></blockquote><div><br></div><div>I cannot but agree</div><div><br></div><div>Paolo<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="auto"><div dir="auto"><div><br></div>
Regards <br><br><div dir="auto">-- <br>Lorenzo Paulatto CNRS /SU <br>Written on a virtual keyboard with real fingers</div></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sun, 31 Mar 2019, 05:23 Nicolas Leconte, <<a href="mailto:lecontenicolas0@uos.ac.kr" target="_blank">lecontenicolas0@uos.ac.kr</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr">Dear QE users,<div><br></div><div>I have recently used the ev.x executable to obtain the equation of state from my energy versus volume data.</div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>Am I missing something as on how to use the output from this script. It feels like too good a coincidence.</div><div><br></div><div>Now, I wonder what precludes me from using this chisq as a prefactor to the bulk modulus B0 (<a href="https://en.wikipedia.org/wiki/Birch%E2%80%93Murnaghan_equation_of_state" rel="noreferrer" target="_blank">https://en.wikipedia.org/wiki/Birch%E2%80%93Murnaghan_equation_of_state</a>) to obtain a proper agreement with the numerically derived curve.</div><div><br></div><div>Regards,</div><div>Nicolas</div></div>
_______________________________________________<br>
users mailing list<br>
<a href="mailto:users@lists.quantum-espresso.org" rel="noreferrer" target="_blank">users@lists.quantum-espresso.org</a><br>
<a href="https://lists.quantum-espresso.org/mailman/listinfo/users" rel="noreferrer noreferrer" target="_blank">https://lists.quantum-espresso.org/mailman/listinfo/users</a></blockquote></div>
_______________________________________________<br>
users mailing list<br>
<a href="mailto:users@lists.quantum-espresso.org" target="_blank">users@lists.quantum-espresso.org</a><br>
<a href="https://lists.quantum-espresso.org/mailman/listinfo/users" rel="noreferrer" target="_blank">https://lists.quantum-espresso.org/mailman/listinfo/users</a></blockquote></div><br clear="all"><br>-- <br><div dir="ltr" class="gmail-m_-4927915402865750843gmail_signature"><div dir="ltr"><div><div dir="ltr"><div>Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,<br>Univ. Udine, via delle Scienze 208, 33100 Udine, Italy<br>Phone +39-0432-558216, fax +39-0432-558222<br><br></div></div></div></div></div></div>
_______________________________________________<br>
users mailing list<br>
<a href="mailto:users@lists.quantum-espresso.org" target="_blank">users@lists.quantum-espresso.org</a><br>
<a href="https://lists.quantum-espresso.org/mailman/listinfo/users" rel="noreferrer" target="_blank">https://lists.quantum-espresso.org/mailman/listinfo/users</a></blockquote></div>