[Pw_forum] equilibrium lattice constant determination

[sreekar guddeti]

Dear QE users,
i performed 'vc-relax' on GaAs to find out zero temperature lattice
constant. i performed similar calculation of energy vs lattice const(a). If
we consider energy to be stored in the field (which is present in the volume
of the cell) then minimum of Energy/Vol vs  'a' should give the result. But
instead the minimum of E vs 'a' is giving a result(the earlier graph isnt
giving a minimum at all) in accordance with that given by vc-relax. I would
be grateful to be clarified of the understanding of the physics of this
situation.

sincerely,
-- 
Sreekar Guddeti
IIT Bombay
BTech + MTech
Physics Dept
IIT Bombay
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[Pw_forum] equilibrium lattice constant determination

[Emine Kucukbenli]

Dear Sreekar,
Could you please clarify why, in your opinion, energy/(unit cell
volume) vs (unit cell volume) should give the minimum but not energy
vs (unit cell volume)? Since you are trying to find the gs lattice
constant, I am assuming you consider harmonic approximation around gs
lattice parameter. So You'll expect to see a parabola in energy vs
volume graph, as energy is proportional to (vol-v0)^2 around
equilibrium(v0). In principle if you draw energy/vol vs vol, you
should see energy/vol proportional to vol+v0^2/vol, which I think
should still have a minimum.But why would you do that, since you are
looking for the configuration with minimum energy?
If this doesnt satisfy you, you can always use an equation of state fitting.
Thats is my understanding of the issue, I'd be more than glad if
somebody corrects me.
emine kucukbenli, Sissa, Italy



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