[Wien] Query regarding elastic constants of SnSe2

[Fecher, Gerhard]

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