<div dir="ltr">Dear Prof. Fecher,<br><div><br></div><div> Thank you for your elaborate reply Sir. It will be very helpful.</div><div><br></div><div>with regards,</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, 12 Aug 2024 at 14:11, Fecher, Gerhard <<a href="mailto:fecher@uni-mainz.de">fecher@uni-mainz.de</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Dear Morteza<br>
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,<br>
but IRElast does not make use of it !<br>
Your answer <br>
"Hexagonal compounds have five independent elastic constants (C66=(c11-c12)/2)"<br>
does not fit to the question, as the compound belongs to a trigonal (NOT hexagonal) crystal system, so what ?<br>
(Also note that the distortions for trigonal and tetragonal systems may be different, even though the number of cij is the same.)<br>
<br>
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 !<br>
The default number of distortions needs to be much larger than 5 otherwise the polynomial fits will have a large uncertainty <br>
(see remarks on ElaStic below and my previous answers) this can be tested easily when calculating standard deviations and errors .<br>
<br>
Dear Shamik,<br>
here is a more detailed answer, please read and understand it<br>
Your queries were;<br>
(1) I have obtained c55 (with 5 constants) while literature shows c44 & c66<br>
(with 6 constants). Why?<br>
(2) There is a big mismatch in the value of C33. Why?<br>
<br>
Answer (1)<br>
Both is wrong, SnSe2 should have 6 (not only 5) independent coefficients and c66=(c11-c12)/2 is not independent.<br>
But that's not all directly your fault, see above, and you should have asked why c14 is not calculated, however c55=c44 !<br>
Answer (2)<br>
see answer (1), and nobody knows the details about what you have done.<br>
<br>
You may tell me that you used a different structure, but then the comparison to literature is nonsense.<br>
<br>
You mentioned the following values:<br>
c11=94.575, c12=27.192, c13=23.2555, c33=102.7 and c55=22.384<br>
from literature without reference<br>
c11=111, c12=33.39, c13=33.08, c33=22.63, c44=44.48, c66=38.80<br>
these values are obviously taken from Y. Javed et al., Int J. Thermophysics 43 (2021)<br>
and were calculated with VASP (Monkhorst-Pack, 7x7x5, Perdew-Zunger LDA)<br>
I guess the authors have known that they need 6 different cij <br>
and used only the 6 largest one and this results in nonsense (see below). <br>
<br>
For crystal systems see <a href="https://en.wikipedia.org/wiki/Space_group" rel="noreferrer" target="_blank">https://en.wikipedia.org/wiki/Space_group</a><br>
check also for difference between crystal family and crystal system<br>
hexagonal systems have a 6-fold, and trigonal have a 3-fold symmetry.<br>
Trigonal crystal systems may belong to rhombohedral (143-148) or hexagonal (149-167) lattice systems.<br>
see <a href="https://en.wikipedia.org/wiki/Crystal_system#Crystal_classes" rel="noreferrer" target="_blank">https://en.wikipedia.org/wiki/Crystal_system#Crystal_classes</a><br>
<br>
>From J.F. Nye Physical Properties Of Crystals, Their Representation by Tensors and Matrices, Oxford (1985)<br>
one finds from Table 9 for the number of independent cij or sij:<br>
isotropic 2<br>
cubic 3<br>
hexagonal 5<br>
trigonal 6 or 7, depending on space group<br>
tetragonal 6 or 7, depending on space group<br>
orthorhombic 9<br>
monoclinic 13 depending on orientation<br>
triclinic 21 <br>
see also R. Golesorkhtabar et al, Comp Phys Comm 184 (2013), in particular Tables 1 and 4.<br>
Here you will find also a detailed description of the deformation types and magnitudes.<br>
This paper will answer also the follow up question about the order of the fit (e.g. Figs. 3 and 4).<br>
Note that they used 51 distortions and 6th order polynomial fits !<br>
<br>
>From the Materials Project webpage one finds the following elastic tensors <br>
<a href="https://legacy.materialsproject.org/materials/mp-665/" rel="noreferrer" target="_blank">https://legacy.materialsproject.org/materials/mp-665/</a><br>
Space group: P -3m1 [164]<br>
Point Group: -3m<br>
Crystal System: trigonal<br>
see also Pearsons Crystal Database for the structure of SnSe2. <br>
<br>
Gamma, 10x10x6, PBE <br>
Stiffness Tensor Cij (GPa)<br>
85 21 3 0 0 0<br>
21 85 3 -0 0 0<br>
3 3 5 0 0 0<br>
0 -0 0 2 0 0<br>
0 0 0 0 2 0<br>
0 0 0 0 0 32<br>
Compliance Tensor Sij<br>
12.8 -3 -5.9 -4.1 0 0<br>
-3 12.8 -5.9 4.1 0 0<br>
-5.9 -5.9 229.1 0 0 0<br>
-4.1 4.1 0 611.4 0 0<br>
0 0 0 0 611.4 -8.1<br>
0 0 0 0 -8.1 31.5<br>
from the non vanshing s14 it is clear that also c14 is not vanishing !!<br>
<br>
<br>
AS IRElast does not calculate for trigonal systems one finds that<br>
VASP delivers in detail (note the different k-mesh)<br>
Monkhorst-Pack, 8x8x4, PBE<br>
<br>
from OUTCAR <br>
NOTE THIS IS in kBar NOT in GPa, and some coefficients have different indices !!<br>
TOTAL ELASTIC MODULI (kBar)<br>
Direction XX YY ZZ XY YZ ZX<br>
--------------------------------------------------------------------------------<br>
XX 1009.9710 280.4181 153.8533 0.1441 -4.8747 -0.0733<br>
YY 280.4181 1012.6904 153.8225 2.7466 4.8397 -0.5768<br>
ZZ 153.8533 153.8225 281.1224 0.0784 0.0167 -0.7403<br>
XY 0.1441 2.7466 0.0784 366.7170 0.2335 -4.9295<br>
YZ -4.8747 4.8397 0.0167 0.2335 95.3172 -0.4235<br>
ZX -0.0733 -0.5768 -0.7403 -4.9295 -0.4235 91.8435<br>
--------------------------------------------------------------------------------<br>
<br>
<br>
from vaspkit <br>
Read Elastic Constants from the ELASTIC_TENSOR.in file if it exists (GPa).<br>
100.9971 28.0418 15.3853 -0.4875 -0.0073 0.0144<br>
28.0418 101.2690 15.3822 0.4840 -0.0577 0.2747<br>
15.3853 15.3822 28.1122 0.0017 -0.0740 0.0078<br>
-0.4875 0.4840 0.0017 9.5317 -0.0423 0.0234<br>
-0.0073 -0.0577 -0.0740 -0.0423 9.1844 -0.4929<br>
0.0144 0.2747 0.0078 0.0234 -0.4929 36.6717<br>
<br>
constants for the trigonal crystal system with space group 149 to 167, here R -3m (164)<br>
c11 c12 c13 c14 0 0.00000<br>
.. c22=c11 c23=c13 c24=-c14 0.00000 0.00000<br>
.. .. c33 0 0.00000 0.00000<br>
.. .. .. c44 0.00000 0.00000<br>
.. .. .. .. c55=c44 c56=c14<br>
.. .. .. .. .. c66=(C11-c12)/2<br>
<br>
using symmetry the tensor reduces to<br>
Elastic Constants (GPa).<br>
101.13305 28.04180 15.38375 -0.48813 0.00000 0.00000<br>
.. 101.13305 15.38375 0.48813 0.00000 0.00000<br>
.. .. 28.11220 0.00000 0.00000 0.00000<br>
.. .. .. 9.35805 0.00000 0.00000<br>
.. .. .. .. 9.35805 -0.48813<br>
.. .. .. .. .. 36.54562<br>
<br>
<br>
<br>
<br>
<br>
Ciao<br>
Gerhard<br>
<br>
DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:<br>
"I think the problem, to be quite honest with you,<br>
is that you have never actually known what the question is."<br>
<br>
====================================<br>
Dr. Gerhard H. Fecher<br>
Institut of Physics<br>
Johannes Gutenberg - University<br>
55099 Mainz<br>
________________________________________<br>
Von: Wien [<a href="mailto:wien-bounces@zeus.theochem.tuwien.ac.at" target="_blank">wien-bounces@zeus.theochem.tuwien.ac.at</a>] im Auftrag von morteza jamal via Wien [<a href="mailto:wien@zeus.theochem.tuwien.ac.at" target="_blank">wien@zeus.theochem.tuwien.ac.at</a>]<br>
Gesendet: Sonntag, 11. August 2024 07:30<br>
An: Shamik Chakrabarti; <a href="mailto:wien@zeus.theochem.tuwien.ac.at" target="_blank">wien@zeus.theochem.tuwien.ac.at</a><br>
Cc: morteza jamal<br>
Betreff: Re: [Wien] Query regarding elastic constants of SnSe2<br>
<br>
Hi<br>
Please check parameters ( lattice parameters, XC, kpoint mesh , ...) which you have used for calculations with them in SnSe2 paper.<br>
As i could find<br>
You:<br>
a= 3.89218 Ang, c=6.582611<br>
XC= PBE<br>
<br>
IF your system is Spin-plorized your command run must be: runsp_lapw ....<br>
<br>
paper:<br>
a=3.811 Ang, c=6.137<br>
XC=LDA<br>
Kpoint mesh= 15×15×11<br>
<br>
DId you checked forces on atoms after minimization??<br>
<br>
Hexagonal compounds have five independent elastic constants (C66=(c11-c12)/2)<br>
And<br>
Tetragonal compounds have 6 or 7 independent elastic constants.<br>
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</blockquote></div><br clear="all"><div><br></div><span class="gmail_signature_prefix">-- </span><br><div dir="ltr" class="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div style="font-size:small">Dr. Shamik Chakrabarti</div><div style="font-size:small">Research Fellow </div><div style="font-size:small">Department of Physics</div><div style="font-size:small">Indian Institute of Technology Patna</div><div style="font-size:small">Bihta-801103</div><div style="font-size:small">Patna</div><div style="font-size:small">Bihar, India</div></div></div></div></div></div></div></div>