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[zhaohscas]

Hi forks,

I use the pwscf 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 at yahoo.com.cn> 
Xinjiang Technical Institute of Physics and Chemistry
Chinese Academy of Sciences 
GnuPG DSA: 0xD10849
2008-07-12