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["Anglade Pierre-Matthieu" <anglade@gmail.com>]

Hi,

As long as your formula are correct they will give the exact same
result in the limit that your computation is perfectly converged. You
can test very easily the convergence by increasing your k-point
samplign and ecut. If the two computation of bulk modulus tend to the
same result when you increase absolute convergence then your formula
are correct. Ottherwise you can be sure there is a flaw.

regards

PMA


2008/7/12 zhaohscas <zhaohscas@yahoo.com.cn>:
> Hi forks,
>
> I use the abinit to compute bulk modulus of BaHfO_3, I obtaibed the the 
> volume and energy under zero pressure first, then I compute the several 
> volume and energy dataset of BaHfO_3 under 2.5GPa, 5.0GPa, 7.5GPa, 10GPa, 
> 20GPa, and 30GPa respectively.
>
> Then, I use the dataset to compute bulk modulus.  Due to the formulae of  
> P-V and E-V Birch�CMurnaghan equation of state are all can be used to do 
> this, in order to comparison, both the P-V and E-V Birch�CMurnaghan 
> equation of state are used.  But, I found that there're  great difference 
> between the result obtained from fitting P-V Birch�CMurnaghan equation of 
> state and that of the E-V Birch�CMurnaghan equation of state.  I also found 
> that the result from P-V fitting is more close to the reports value in the 
> literatures.
>
> Furthermore, I also have a puzzle about the form of the formulae of  P-V 
> and E-V Birch�CMurnaghan equation of state, in detail, I've read from the 
> different literatures that there are two forms of the P-V Birch�CMurnaghan 
> equation of state:
>
> a)  P(V) = \frac{{3B_0 }}{2}\left[ {\left( {\frac{{V_0 }}{V}} 
> \right)^{\frac{7}{3}}  - \left( {\frac{{V_0 }}{V}} \right)^{\frac{5}{3}} } 
> \right]\left\{ {1 + \frac{3}{4}\left( {B_0^'  - 4} \right)\left[ {\left( 
> {\frac{{V_0 }}{V}} \right)^{\frac{2}{3}}  - 1} \right]} \right\}
>
> b)  P(V) = \frac{{3B_0 }}{2}\left[ {\left( {\frac{{V_0 }}{V}} 
> \right)^{\frac{7}{3}}  - \left( {\frac{{V_0 }}{V}} \right)^{\frac{5}{3}} } 
> \right]\left\{ {1 + \frac{3}{4}\left( {4 - B_0^'} \right)\left[ {\left( 
> {\frac{{V_0 }}{V}} \right)^{\frac{2}{3}}  - 1} \right]} \right\}
>
> Where, the B_0 and the B_0^' are the bulk modulus and its pressure 
> derivative respectively.
>
> Which of the above is correct?
>
> As for the E-V Birch�CMurnaghan equation of state, I read the following 
> form:
>
> E(V) = E_0  + \frac{{9V_0 B_0 }}{{16}}\left\{ {\left[ {\left( {\frac{{V_0 
> }}{V}} \right)^{\frac{2}{3}}  - 1} \right]^3 B_0^'  + \left[ {\left( 
> {\frac{{V_0 }}{V}} \right)^{\frac{2}{3}}  - 1} \right]^2 \left[ {6 - 
> 4\left( {\frac{{V_0 }}{V}} \right)^{\frac{2}{3}} } \right]} \right\}
>
> Is this right or not?
>
> Who can give me some hints?
>
> Sincerely yours,
> --------------
> Hongsheng Zhao <zhaohscas@yahoo.com.cn>
> Xinjiang Technical Institute of Physics and Chemistry
> Chinese Academy of Sciences
> GnuPG DSA: 0xD10849
> 2008-07-12
>



-- 
Pierre-Matthieu Anglade