[Wien] possible pressure-induced transition(find DeltaG)

[Stefaan Cottenier]

This is a typical numerical annoyance -- the enthalpy curves of both 
phases are shallowly intersecting. And plotting the difference as a 
function of pressure is tricky because you don't have the same pressure 
grid for both phases.

However, as you have all parameters of the (Birch-)Murnaghan equation at 
this stage, you can find the volume and the energy at any given pressure 
(for the Murnaghan equation, you even have the analytical expression at 
Wikipedia -- http://en.wikipedia.org/wiki/Murnaghan_equation_of_state). 
And therefore also the enthalpy at that pressure. Hence, you can 
construct the enthalpies for both phases on the same pressure grid.

Stefaan

On 14/03/2014 5:43, MAHDI SALMANI HIRMAND wrote:
> Dear Prof. Blaha,
> I am studying possible pressure-induced transition for some compounds.
> stefaan guide me how i can calculate Gibbs energy and find the pressure.
>
> He told me "you can easily calculated the enthalpy H=E+PV using
> case.eosfit and case.eosfitb. Repeat for the second phase, and see at
> which pressure the two enthalpy curves intersect."
>
> i did it but the two enthalpy curves are almost the same and it is
> difficult finding the position which two enthalpy curves intersect.
> usually in paper they find difference of enthalpy energy from two phases
> and therefore the pressure which in it DeltaG or DeltaH=0
>
> i would like to use this method. but when i check case.eosfitb file for
> two phases i see the values of pressure are different for example
> case1.eosfitb                case2.eosfitb
>   V1    E1     8.4                 V2     E2     8.9
>    "       "        8.1                  "         "       8.7
>    "       "        8.01               "          "       8.5
>                                             "          "       8.08
> can you guide me how i can find DeltaG when the value of pressure are
> different?
>
> Sincerely
> Mahdi
>
>
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