[QE-users] Which celldm(i) to choose for DOS

[Giovanni Cantele]

Dear Sudip,

I’m not sure I’ve fully understood your question. You are saying that, after a vc-relax calculation, you use the final output lattice parameter(s) in a single scf run,
and you obtain a different pressure, aren’t you?

As far as I remember, during vc-relax the code uses a fixed number of plane waves (because it cannot change the basis set during a single calculation).
On the other hand, in a single scf run the number of plane waves depends on the lattice parameter. So, it might happen that the basis set of the scf run is different from
that of the vc-relax run, because the latter is setup based on the initial cell, and not the last one that is unknown. So maybe you could compare the two basis sets, and then
try to make for example the scf run with the vc-relax basis set, in place of the default one, just to check that you obtain exactly the same results. In any case, I think that if this is 
the case the differences you experience in the lattice parameter should be negligible or small.

Giovanni

PS by the way, after writing this message I’ve find the FAQ addressing just you issue!
http://www.quantum-espresso.org/resources/faq/self-consistency#6.11

> On 16 May 2018, at 09:25, Sudip Kumar Mondal <sudipkm1990 at gmail.com> wrote:
> 
> Hi all ,
>      
> I'm currently using QE 5.4.0. & I have a general question regarding the calculation of density of states (LDOS & PDOS).
> 
> Before estimating any physical observable we optimize (relax/vc-relax or sometimes both) the structure in hand to obtain accurate lattice parameter(s) corresponding to zero kBar pressure. Optimizations w.r.t. all other parameters (k-point grid , ecutwfc , ecutrho etc.)  are also performed.
> However for a single scf run with the lattice parameter(s) corresponding to zero pressure as obtained from relax/vc-relax , yields some different pressure other than zero kBar.  In my case then I ran a series of single scf calculations for converging to zero pressure by varying the lattice parameter. The scf convergence to zero pressure was successful. But the lattice parameter  turned out be smaller than its zero pressure equivalent obtained from relaxation by ~ 0.9 bohr.
> 
> Now, my question is whether I should proceed to calculate DOS with the value of the lattice parameter(s) corresponding to zero pressure obtained from relaxation or with the same obtained from the series of single scfs?
> 
> 
> Regards.
> -- 
> Sudip Kumar Mondal
> DST INSPIRE Fellow
> High Pressure & Temperature Laboratory
> Dept. Of Physics/Geological Sc.
> Jadavpur University.
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-- 

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: http://people.fisica.unina.it/~cantele

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