[Wien] Query regarding elastic constants of SnSe2

[shamik chakrabarti]

Dear Sir,

               In the output-order file there are 3 types of outputs. One
is for order of fit 2, another is order of fit 3 and the last one is order
of fit is 4. My query which output among those three should be considered?

with regards,

On Tue, 6 Aug 2024 at 13:37, Fecher, Gerhard <fecher at uni-mainz.de> wrote:

> Why runsp_lapw, isn't SnSe2 a semiconductor without magnetic order and a
> non-spinpolarized calculation is just fine ?
>
> According to Pearsons Crystal Database from 2022, SnSe2 should crystallize
> in P -3m1 (no. 164),
> in its crystal class -3m (also for 3m, 32) one has 6 independent elastic
> constants (11 12 13 14 33 44)
> (dependent are 22=11, 55=44, 23=13, 24=-14, 56=14,  c66 =( c11-c12)/2))
>
> If you use a hexagonal structure, you will have only 5 independent
> constants (11 12 13 33 44)
> in both cases hexagonal and trigonal (also crystal classes 3, -3 with
> seven independent constants) you have c66 =( c11-c12)/2 which is NOT
> independent
>
> As Morteza mentions, Se on 2d has a free z-parameter and thus the position
> needs to be optimized.
> Moreover, when the lattice is distorted, some atoms may occupy sites with
> free parameters even though the original didn't have.
> Therefore you need to minimize the forces and optimize all free positions
> that appear in all structures during the determination of the elastic
> constants.
> Usually one compares the elastic properties with fixed and relaxed
> positions. The differences in the equation of state p-V-E curves can be
> attributed to something like an intenal pressure as in real gases cpmpared
> to ideal ones.
>
> Did you use the correct structure and space group ?
> On the materials explorer project webpage you will find the matrix with 6
> independent values for the stiffness or compliance tensors.
>
> Everything one needs to know is provided in textbooks !
>
> Ciao
> Gerhard
>
> DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
> "I think the problem, to be quite honest with you,
> is that you have never actually known what the question is."
>
> ====================================
> Dr. Gerhard H. Fecher
> Institut of Physics
> Johannes Gutenberg - University
> 55099 Mainz
> ________________________________________
> Von: Wien [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von
> morteza jamal via Wien [wien at zeus.theochem.tuwien.ac.at]
> Gesendet: Montag, 5. August 2024 20:24
> An: shamik15041981 at gmail.com; wien at zeus.theochem.tuwien.ac.at
> Cc: morteza jamal
> Betreff: Re: [Wien] Query regarding elastic constants of SnSe2
>
> Dear  Shamik,
> As i checked your scf files for C33 calculation, I saw there are forces on
> your atoms (atom 2) and you must relax your stuct files ( runsp_lapw  -min
> .....  or  min_lapw " runsp_lapw ....".
>
> Try to converge  "Energy,  Charge and force"
> runsp_lapw  -min ..... -ec 0.0001 -cc ... -fc ...
>
>
> With best,
> Morteza
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-- 
Dr. Shamik Chakrabarti
Research Fellow
Department of Physics
Indian Institute of Technology Patna
Bihta-801103
Patna
Bihar, India
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