Dear Houcine,
First of all, which exchange-correlation functional are you using? If you are
using LDA, that may explain the underestimation of the lattice constants.
Taking the experimental lattice and relaxing the structure without relaxing the
cell ('relax' instead of 'vc-relax') is a valid approach. I personally prefer
to relax both, provided that the level of theory I am using gives the lattice
constants that are accurate enough. The system doesn't look to me like
something to which I would directly apply DFT-D3, but I haven't studied this
type of materials.
Best wishes,
Marko
________________________________
From: users <[email protected]> on behalf of 후신 부지드
<[email protected]>
Sent: Friday, December 20, 2019 2:57:28 PM
To: [email protected]
Subject: [QE-users] Large difference between experimental lattice constant and
the relaxed one w/ and w/o dft-d3
Dear QE users,
I'm working on a layered material (SnS2) slightly doped with transition metals
(TM)
the experimental c lattice constant of this Trigonal system is around 6.88 A as
measured by XRD (slightly less with Mn doping)
DFT calculation without any dispersion force correction gives 6.49 A
by including a dft-d3 correction during the relaxation it is even less ~ 5.95
- Any advice what value should I consider ? It is correct to take the
experimental value without any relaxation ?
- Same inquiry about London dispersion force correction, is it needed to use it
in this case for next steps (scf and nscf caculations) since it is giving a big
difference compared with the experimental value ?
Any help is appreciated!
Best Regards
Houcine BOUZID
Sungkyunkwan University,
Korea.
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