[Pw_forum] Question about P= pressure

[Lorenzo Paulatto]

In data 30 giugno 2009 alle ore 10:12:38, Bipul Rakshit  
<bipulrr at gmail.com> ha scritto:
> 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.

Dear Bipul,
as a preliminary check, please take care that all your calculations are  
well converged, with respect of wavefunctions cutoff (ecutwfc), charge  
density cutoff (ecutrho) and number of k-points. Also note that DFT  
theory, although very powerful, can only predict experimental data up to  
its accuracy. You could try and check the literature for other DFT  
calculation of the same material, in order to have an idea of how accurate  
you can expect your calculation to be.

> 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.

How much is "far less"?

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

Yes, it is.

> 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,)?

I depends on the details of your calculation, if you'll provide you input  
and output files we could have a look and try to spot possible source of  
problems.

Best regards.

-- 
Lorenzo Paulatto
SISSA  &  DEMOCRITOS (Trieste)
phone: +39 040 3787 511
skype: paulatz
www:   http://people.sissa.it/~paulatto/

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