[Pw_forum] equilibrium volume

[Gabriele Sclauzero]

Dear Mohaddeseh

mohaddeseh abbasnejad wrote:
> 
> Dear all
> When I run an scf calculation for the equilibrium volume obtained by the 
> Murnaghan-fit, the stresses are too far from zero.

What does it mean too far? Please specify some numbers, so that we can have an idea of 
what's happening. You should also specify some input file of your system.

> Should i expect this volume to be zero pressure volume, or there is a 
> problem in the calculation?

I think it will be hardly exactly zero (for obvious numerical reasons) unless you specify 
the exact equilibrium lattice constant with many significant digits and use very high 
cutoffs (I think that stress is much harder to converge than total energy differences). 
You should also keep in mind that the parameters resulting from a fit have an error bar 
associated with them (though this might be quite small for the equilibrium lattice 
constant, it may be enough to offset the stress a little bit from zero). Try to sample 
more points around the minimum and fit with a parabola (such that the chi square of the 
fit is very low).



Gabriele


> Thanx for any comment
> 
> -- 
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> 
> Mohaddeseh Abbasnejad,
> Room No. 323, Department of Physics,
> University of Tehran, North Karegar Ave.,
> Tehran, P.O. Box: 14395-547- IRAN
> Tel. No.: +98 21 6111 8634  & Fax No.: +98 21 8800 4781
> Cellphone: +989177317514
> E-Mail:     m.abbasnejad at gmail.com <mailto:m.abbasnejad at gmail.com>
> Website:  http://physics.ut.ac.ir
> 
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| Gabriele Sclauzero, PhD Student                  |
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