[Wien] Query regarding elastic constants of SnSe2

Pranjal Nandi pnandi at ub.edu
Mon Aug 12 15:23:06 CEST 2024


Aladai !

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From: Wien <wien-bounces at zeus.theochem.tuwien.ac.at> on behalf of shamik chakrabarti <shamik15041981 at gmail.com>
Sent: Monday, August 12, 2024 3:20:50 PM
To: A Mailing list for WIEN2k users <wien at zeus.theochem.tuwien.ac.at>
Subject: Re: [Wien] Query regarding elastic constants of SnSe2

Dear Prof. Fecher,

                        Thank you for your elaborate reply Sir. It will be very helpful.

with regards,

On Mon, 12 Aug 2024 at 14:11, Fecher, Gerhard <fecher at uni-mainz.de<mailto:fecher at uni-mainz.de>> wrote:
Dear Morteza
It is interesting that your paper J All Comp 735 (2018) 569-579 gives the correct answer for trigonal compounds namely 6 cij for the present case,
but IRElast does not make use of it !
Your answer
"Hexagonal compounds have five independent elastic constants  (C66=(c11-c12)/2)"
does not fit to the question, as the compound belongs to a trigonal (NOT hexagonal) crystal system, so what ?
(Also note that the distortions for trigonal and tetragonal systems may be different, even though the number of cij is the same.)

Your script should not look for the H lattice but for the space group, which it finds correctly from the struct file as I tested !
The default number of distortions needs to be much larger than 5 otherwise the polynomial fits will have a large uncertainty
(see remarks on ElaStic below and my previous answers) this can be tested easily when calculating standard deviations and errors .

Dear Shamik,
here is a more detailed answer, please read and understand it
Your queries were;
(1) I have obtained c55 (with 5 constants) while literature shows c44 & c66
(with 6 constants). Why?
(2) There is a big mismatch in the value of C33. Why?

Answer (1)
Both is wrong, SnSe2 should have 6 (not only 5) independent coefficients and c66=(c11-c12)/2 is not independent.
But that's not all directly your fault, see above, and you should have asked why c14 is not calculated, however c55=c44 !
Answer (2)
see answer (1), and nobody knows the details about what you have done.

You may tell me that you used a different structure, but then the comparison to literature is nonsense.

You mentioned the following values:
c11=94.575, c12=27.192, c13=23.2555, c33=102.7 and c55=22.384
from literature without reference
c11=111, c12=33.39, c13=33.08, c33=22.63, c44=44.48, c66=38.80
these values are obviously taken from Y. Javed et al., Int J. Thermophysics 43 (2021)
and were calculated with VASP (Monkhorst-Pack, 7x7x5, Perdew-Zunger LDA)
I guess the authors have known that they need 6 different cij
and used only the 6 largest one and this results in nonsense (see below).

For crystal systems see https://en.wikipedia.org/wiki/Space_group
check also for difference between crystal family and crystal system
hexagonal systems have a 6-fold, and trigonal have a 3-fold symmetry.
Trigonal crystal systems may belong to rhombohedral (143-148) or hexagonal (149-167) lattice systems.
see https://en.wikipedia.org/wiki/Crystal_system#Crystal_classes

>From J.F. Nye Physical Properties Of Crystals, Their Representation by Tensors and Matrices, Oxford (1985)
one finds from Table 9 for the number of independent cij or sij:
isotropic    2
cubic        3
hexagonal    5
trigonal     6 or 7, depending on space group
tetragonal   6 or 7, depending on space group
orthorhombic 9
monoclinic  13 depending on orientation
triclinic   21
see also R. Golesorkhtabar et al, Comp Phys Comm 184 (2013), in particular Tables 1 and 4.
Here you will find also a detailed description of the deformation types and magnitudes.
This paper will answer also the follow up question about the order of the fit (e.g. Figs. 3 and 4).
Note that they used 51 distortions and 6th order polynomial fits !

>From the Materials Project webpage one finds the following elastic tensors
https://legacy.materialsproject.org/materials/mp-665/
Space group:     P -3m1 [164]
Point Group:     -3m
Crystal System:  trigonal
see also Pearsons Crystal Database for the structure of SnSe2.

Gamma, 10x10x6, PBE
Stiffness Tensor Cij (GPa)
85      21      3       0       0       0
21      85      3       -0      0       0
3       3       5       0       0       0
0       -0      0       2       0       0
0       0       0       0       2       0
0       0       0       0       0       32
Compliance Tensor Sij
12.8    -3      -5.9    -4.1    0       0
-3      12.8    -5.9    4.1     0       0
-5.9    -5.9    229.1   0       0       0
-4.1    4.1     0       611.4   0       0
0       0       0       0       611.4   -8.1
0       0       0       0       -8.1    31.5
from the non vanshing s14 it is clear that also c14 is not vanishing !!


AS IRElast does not calculate for trigonal systems one finds that
VASP delivers in detail (note the different k-mesh)
Monkhorst-Pack, 8x8x4, PBE

from OUTCAR
NOTE THIS IS in kBar NOT in GPa, and some coefficients have different indices !!
 TOTAL ELASTIC MODULI (kBar)
 Direction    XX          YY          ZZ          XY          YZ          ZX
 --------------------------------------------------------------------------------
 XX        1009.9710    280.4181    153.8533      0.1441     -4.8747     -0.0733
 YY         280.4181   1012.6904    153.8225      2.7466      4.8397     -0.5768
 ZZ         153.8533    153.8225    281.1224      0.0784      0.0167     -0.7403
 XY           0.1441      2.7466      0.0784    366.7170      0.2335     -4.9295
 YZ          -4.8747      4.8397      0.0167      0.2335     95.3172     -0.4235
 ZX          -0.0733     -0.5768     -0.7403     -4.9295     -0.4235     91.8435
 --------------------------------------------------------------------------------


from vaspkit
Read Elastic Constants from the ELASTIC_TENSOR.in file if it exists (GPa).
   100.9971    28.0418    15.3853    -0.4875    -0.0073     0.0144
    28.0418   101.2690    15.3822     0.4840    -0.0577     0.2747
    15.3853    15.3822    28.1122     0.0017    -0.0740     0.0078
    -0.4875     0.4840     0.0017     9.5317    -0.0423     0.0234
    -0.0073    -0.0577    -0.0740    -0.0423     9.1844    -0.4929
     0.0144     0.2747     0.0078     0.0234    -0.4929    36.6717

constants for the trigonal crystal system with space group 149 to 167, here R -3m (164)
   c11         c12          c13          c14        0           0.00000
    ..         c22=c11      c23=c13      c24=-c14   0.00000     0.00000
    ..          ..          c33          0          0.00000     0.00000
    ..          ..          ..           c44        0.00000     0.00000
    ..          ..          ..          ..          c55=c44     c56=c14
    ..          ..          ..          ..          ..          c66=(C11-c12)/2

using symmetry the tensor reduces to
Elastic Constants (GPa).
   101.13305    28.04180    15.38375    -0.48813     0.00000     0.00000
    ..         101.13305    15.38375     0.48813     0.00000     0.00000
    ..          ..          28.11220     0.00000     0.00000     0.00000
    ..          ..          ..           9.35805     0.00000     0.00000
    ..          ..          ..          ..           9.35805    -0.48813
    ..          ..          ..          ..          ..          36.54562





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<mailto:wien-bounces at zeus.theochem.tuwien.ac.at>] im Auftrag von morteza jamal via Wien [wien at zeus.theochem.tuwien.ac.at<mailto:wien at zeus.theochem.tuwien.ac.at>]
Gesendet: Sonntag, 11. August 2024 07:30
An: Shamik Chakrabarti; wien at zeus.theochem.tuwien.ac.at<mailto:wien at zeus.theochem.tuwien.ac.at>
Cc: morteza jamal
Betreff: Re: [Wien] Query regarding elastic constants of SnSe2

Hi
Please check  parameters ( lattice parameters, XC, kpoint mesh , ...) which you have used for calculations with them in SnSe2 paper.
As i could find
You:
a= 3.89218 Ang, c=6.582611
XC= PBE

IF your system is Spin-plorized  your command run must be: runsp_lapw ....

paper:
a=3.811 Ang, c=6.137
XC=LDA
Kpoint mesh= 15×15×11

DId you checked forces on atoms after minimization??

Hexagonal compounds have five independent elastic constants  (C66=(c11-c12)/2)
And
Tetragonal compounds have 6 or 7 independent elastic constants.
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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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