[Pw_forum] Fw : ev.x procedures

[Sandro Scandolo]

Dear Bernard,

>
> So, if I want to plot the P-V diagram, I can just calculate the 
> pressure for each volume using the EOS,since the fitting parameters 
> are already known. Is it correct?
>

Yes, correct.

>
> Also, when implementing the fitting procedure iteratively with ev.x, 
> on has to choose between the Murnagham, Birch 1 and Birch 2. According 
> to me, there is nothing in the litterature known as Birch 1 and Birch 
> 2. I thing they refer to Second order and Third order Birch-Murnagham 
> EOS. I would like you to clarify it for me.
>

I presume so, but you may wish to check what's inside ev.f90 and let us 
know. :-)

Regards,
Sandro
 


>
>
>
> --- En date de : *Mar 2.2.10, Sandro Scandolo /<scandolo at ictp.it>/* a 
> écrit :
>
>
>     De: Sandro Scandolo <scandolo at ictp.it>
>     Objet: Re: [Pw_forum] Fw : ev.x procedures
>     À: "PWSCF Forum" <pw_forum at pwscf.org>
>     Date: Mardi 2 Février 2010, 16h19
>
>     Dear Bertrand,
>
>     pressure is not an input of your pw calculation. It is an output
>     of the
>     ev.x calculation.
>
>     What ev.x does is to fit the results of your E(V) pw calculations
>     to a
>     chosen EOS functional form, with a number of parameters. Most
>     functional
>     forms (e.g. Birch-Murnaghan) include the parameters K and K' (see
>     http://en.wikipedia.org/wiki/Birch-Murnaghan_equation_of_state ).
>     P(V)
>     can then be calculated from the chosen E(V) (and the fitted
>     parameters)
>     as P(V)=-dE(V)/dV .
>
>     Notice that the P(V) calculated form the EOS with the fitted
>     parameters
>     may be slightly different with respect to the one calculated by
>     pw.x at
>     the volumes where you did your pw.x calculations. But this is another
>     story (related to
>     http://www.quantum-espresso.org/wiki/index.php/Methodological_Background#Stress
>
>     ).
>
>     So, coming to your question, K' is not obtained by finite
>     differences,
>     but rather directly as a result of the E(V) fit.
>
>     Sandro
>
>
>     Bertrand SITAMTZE wrote:
>     >
>     >
>     > --- En date de : *Mar 2.2.10, Bertrand SITAMTZE
>     /<siyouber at yahoo.fr </mc/compose?to=siyouber at yahoo.fr>>/*
>     > a écrit :
>     >
>     >
>     >     De: Bertrand SITAMTZE <siyouber at yahoo.fr
>     </mc/compose?to=siyouber at yahoo.fr>>
>     >     Objet: ev.x procedures
>     >     À: "pw code" <pw_forum at pwscf.org
>     </mc/compose?to=pw_forum at pwscf.org>>
>     >     Date: Mardi 2 Février 2010, 11h04
>     >
>     >     (SORRY, THERE WAS A SMALL MISTAKE IN MY PREVIOUS MESSAGE)
>     >
>     >
>     >     Dear all,
>     >
>     >     As I know, scf and nscf in pw.x are with zero pressure (please
>     >     correct me if I am wrong). Thus, when evaluating the EOS
>     >     parameters with ev.x, the code uses the E vs VOLUME data to
>     >     evaluate the bulk modulus throught the second derivative.
>     But, how
>     >     does the code proceed in order to evaluate the pressure
>     derivative
>     >     of the bulk modulus K', without varing the pressure? If
>     otherwise,
>     >     can somebody tell me the range of pressure used to evaluate K'?
>     >
>     >     Please, note that K' is defined as K'=(dK/dP) at P=0.
>     >
>     >
>     >     Thanks very much for your answers.
>     >
>     >
>     >
>     >     ***************************************
>     >     Bertrand SITAMTZE YOUMBI
>     >     Laboratory of Material Sciences
>     >     University of Yaounde I-Cameroon
>     >     *************************************
>     >
>     >
>     >
>     >
>     ------------------------------------------------------------------------
>     >
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>     >   
>
>     -- 
>     Sandro Scandolo, ICTP | http://www.ictp.it/~scandolo
>     <http://www.ictp.it/%7Escandolo>
>
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-- 
Sandro Scandolo, ICTP | http://www.ictp.it/~scandolo