[Pw_forum] regarding bulk modulus

[Paolo Giannozzi]

On Apr 10, 2010, at 10:22 , Shyam Khambholja wrote:

> I have performed total energy calculation for Al as a function of  
> lattice parameter.
> then i fit those data to murnaghan equation of state and got bulk  
> modulus. when i
> use method of hydrostatic strain and got bulk modulus, it is much  
> smaller than the
> one obtained by fitting E-a curve to murnaghan eos. So, which one  
> is more reliable,
> since, bulk modulus obtained by fitting to mur. eos is quite near  
> to experimental data.
> and which one should be used to determine other elastic stiffness  
> constant ?

this is one of the first thing anybody involved in calculations in  
solids should know.
Fitting to an equation of state converges more quickly with the plane- 
wave basis set.
If however you find large differences between the various methods,  
either you did
something wrong, or you are very fare from convergence wrt PW

P.
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Paolo Giannozzi, Dept of Physics, University of Udine
via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222