[Pw_forum] Vacuum for non-periodic structure

[Giovanni Cantele]

Hi, if you want to study a nanowire, I do not see the reason why to optimise the unit cell along x and y directions.
Either freeze these two degrees of freedom in vc-relax or, much simpler, change c parameter (lattice parameter along the z direction)
and evaluate E(c) (total energy as a function of c). A parabolic fit will help you in finding the lowest energy configuration.

Generally speaking, to establish whether the vacuum is enough, calculate in at least one case the property you are interested in for two different
values of the vacuum and see if any appreciable change is observed. Another check is that the band structure should be flat (no dispersion)
along x and y

Giovanni



> On 29 Nov 2017, at 06:28, Anik Mondol <anikmondol1206 at gmail.com> wrote:
> 
> Dear Users,
> I am doing a DFT calculation of GaN nanowires, While designing the nanowire (approximately 10 A diameter) , I had added around 10 A of vacuum on x and y direction. Now I have performed a vc-relax calculation before band calculation, the cell volume has shrunk . The vacuum has decreased . Now to make sure the results don't be affected by the identical replicas , at least  how much vacuum is needed ? Now, that the vacuum has shrunk, will that impose any effect on results in this respects. I mean, How can I be sure this vacuum is enough?
> 
> Mondol Anik Kumar 
> Bangladesh University of Engineering and Technology 
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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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