[Pw_forum] Question about P= pressure

[Axel Kohlmeyer]

On Tue, 2009-06-30 at 13:42 +0530, Bipul Rakshit wrote:
> Dear PWSCF users,
> I am doing calculation on Mg2Si. There I just want to plot Pressure vs
> Volume graph.
> So what i did, first I found the equilibrium lattice constant (P=0.0),
> then I reduce the lattice parameter, so the pressure increases.
> 
> With that Volume vs Pressure data, I plot the graph. But when i
> compared with experimental VvsP, it is not at all matching.
> Infact my volume is far less than the experimental value.

please try to act like a proper scientist and provide exact
details instead of vague descriptions. _how far_ less is your
volume. is it a factor? a few percent? an order of magnitude?

> So my question is, the pressure term which we get at the end of scf
> run, is the pressure on the system?

yes. if it wasn't a lot of calculations and published paper would 
be far off. are you sure you computed the volume of your cell correctly?

> If the pressure is equivalent to the pressure on the system, so why I
> am getting such large deviation (cal. volume is very less than expt,)?

how should anybody know without any details and hard facts?

> Please help me in this matter.

it is very difficult to help somebody making such vague statements.
all one can do is guess and give vague directions. your SCF calculation
may be wrong, you pseudopotentials crap, you calculation of the volume 
wrong, you make have mixed up units, and other things.

cheers,
   axel.



-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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