[QE-users] Which pressure should I report on my work???

[Stefano Baroni]

> On 20 Sep 2020, at 18:25, Robert Molt <r.molt.chemical.physics at gmail.com> wrote:
> 
> If I may ask a question to clarify a point within this thread…I have always had some confusion on exactly why the the # of basis vectors is not conserved step-to-step. This is an implicit aspect to the question being asked, if I understand properly.
> 
> 1.) Why is the number of plane waves during vc-relax steps constant? Why is not held to a constant energy cutoff, like in an energy calculation? 

Because the energy as a function of the unit-cell shape/volume is discontinuous at constant cutoff.

> 
> My understanding is that ecutrho and ecutwfc “constrain” the number of plane waves during a vc-relax calculation; these two variables completely specify the number of allowed plane waves consistent with the initial unit cell volume. Is this correct?
> 
> My understanding is that if you wanted a constant energy cutoff, you have to vary the number of plane waves, since the volume is changing. Why is this undesirable to do? Is it because comparing different numbers of plane waves results in different energies, such that no fair comparison can be made between calculations with different numbers of basis vectors?
> 
> 2.) On this specific question asked by Mohad, I am slightly confused. Is the essence of the problem that the pressure calculation threshold is too low OR is it that you should never use the “properties” other than lattice constants from a vc-relax for the aforementioned reason? In my head, these are seemingly independent issues in how the calculations are working?

As I tried to hint in a previous post, the point here is that a few kbar is an exceedingly small pressure for most purposes. Whether or not this is the case depends on the magnitude of the derivative of the quantity one is interested in with respect to pressure/volume. The relation between the two depends of course on the bulk modulus.

SB

— 
Stefano Baroni -  SISSA, Trieste - http://stefano.baroni.me <http://stefano.baroni.me/>, stefanobaroni (Skype) 

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