[Pw_forum] enthlapy of a system

[Matteo Cococcioni]

Dear Bipul,

the pressure you get as the Tr of the stress tensor can be a little 
inconsistent with the one you would get from the E(V) curve unless you
keep the number of plane waves constant for all the lattice spacing 
(i.e. you change the cutoff with the volume of the unit cell).
The reason is the variational nature of E: for larger cells you get 
smaller Brillouin zones and a larger number of plane waves in the 
calculation that makes E decrease more than "expected" just from the 
increase of lattice spacing.
The stress instead is computed for a given unit cell and a fixed number 
of plane waves.

Probably you can find other messages about this in the forum archive.

Regards,

Matteo




Bipul Rakshit wrote:
> Dear Matteo,
> Got your point and really thanks for the help.
> Just one more question....
> In scf run if we set tstress = .true., then we also get the pressure. 
> And also we have Etot and V, so can we calculate H taking those Etot 
> and V ??
>  
> On Tue, Sep 2, 2008 at 11:02 AM, Matteo Cococcioni <matteo at umn.edu 
> <mailto:matteo at umn.edu>> wrote:
>
>
>
>     Dear Bipul
>
>     I think all you need to do is computing the equation of state for both
>     phases. In other words you choose a series of lattice parameters (that
>     may be different for the two phases) and compute the total energy for
>     each of them. In this case for B1 (NaCl) and B2 (CsCl) you don't
>     need to
>     relax the atomic positions for each lattice parameter as you would
>     do in
>     general. Once you get E(V) for each of the phase, you can use the
>     program ev.x to fit a Murnaghan or a Birch eq. of state in your data.
>      From the equation of state you have chosen you can compute the
>     derivative P=-dE/dV at any V within the interval of your fit. This
>     will
>     be the pressure on your system. for every value of P
>     you will have V and the corresponding E so you will be able to
>     construct
>     E+PV (enthalpy) for the values of P you have.
>
>     hope this helps,
>
>     Matteo
>
>
>
>
>     Bipul Rakshit wrote:
>     > Hello Everyone,
>     > I want to calculate the enthlapy at different pressures of ScAs
>     in B1
>     > and B2 phase, so that i able to find the transition pressure. Can
>     > anybody tell me the correct method to do that?
>     >
>     >
>     > --
>     > Bipul Rakshit
>     > PhD Student,
>     > Barkatullah University,
>     > Bhopal 462026,
>     > MP, India
>     >
>     ------------------------------------------------------------------------
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>
>
> -- 
> Bipul Rakshit
> PhD Student,
> Barkatullah University,
> Bhopal 462026,
> MP, India
> ------------------------------------------------------------------------
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