<div dir="ltr"><div><br></div><div>Thank you Prof. Peter for detailed reply,</div><div>May be I still have some doubts on symmetries and need to read some literature. </div><div>For meanwhile, from literature I came to know that the distortion in my doped structure "change in unit cell ("lattice parameters and angles")" may occur depending on size effects, deviations from the ideal composition and the JTE etc.</div><div><br></div><div>So what I am doing is good and there is no way to preserve the same SG/unit cell. </div><div><br></div><div>I tried to maintain the original structure and got below warnings which support my above understanding:</div><div><br></div><div>1. warning: !!! Unit cell has been reduced.</div><div><br></div><div>------------------------------<wbr>------------------------------<wbr>----------</div><div><br></div><div>Bravais lattice: Tetragonal primitive</div><div><br></div><div> a b c</div><div> 13.98640455 13.98640455 19.77976300 >> the original was 19.77976300</div><div> alpha beta gamma</div><div> 90.00000000 90.00000000 90.00000000</div><br><br>2. SPACE GROUP CONTAINS INVERSION<div><br></div><div>3. ROTDEF - Error<div>0.0u 0.0s 0:00.00 0.0% 0+0k 0+24io 0pf+0w</div><div class="gmail_extra"><br clear="all"><div><div class="gmail_signature"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div><br><font color="#000000">So, if I accept all warnings during initialisation, I get different SGs.</font></div><div><font color="#000000"><br></font></div><div><font color="#000000">Sincerely</font></div><div><font color="#000000">Bhamu </font></div></div></div></div></div></div></div>
<br><div class="gmail_quote">On Tue, May 9, 2017 at 5:22 PM, Peter Blaha <span dir="ltr"><<a href="mailto:pblaha@theochem.tuwien.ac.at" target="_blank">pblaha@theochem.tuwien.ac.at</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">You are mixing up the concepts of space groups and<br>
bravais lattice or unit cell.<br>
<br>
For a certain structural input, there is only ONE space group.<br>
<br>
However, even for one and the same SG, there are often several "settings", which can lead to VERY different unit cells (bravais lattices, lattice vectors) or positions of the atoms within the cell.<br>
<br>
And besides the "standard" settings (which is taken by sgroup), there are often very many different possible settings ("unit cells"), which are equivalent to each other.<br>
<br>
I've mentioned this several times on the list: if you "know" what you are doing, there is in most cases no need to follow the setting as suggested by sgroup (except when the size of the unit cell is reduced).<br>
<br>
Instead, you can very often use the multiplicities as given by nn and use the symmetry operations as given by "symmetry".<br>
<br>
However, you should check if sgroup gives you<br>
a) identical multiplicities<br>
b) the same number of symmetry operations.<br>
c) if symmetry complains, that inversion is present, but you should shift the origin, then you must either take sgroups struct file or shift all positions yourselv (x supercell can do that for you too).<br>
<br>
If this is fulfilled, you do not need to take the struct file from sgroup, which may have changed the unit cell ("lattice parameters and angles") to cope with some standard settings.<br>
<br>
In other words: then you can probably stay with your "cubic" perovskite unit cell (even when you get P1 symmetry, i.e. have only ONE symmetry operation).<br>
<br>
On 05/09/2017 01:38 PM, Dr. K. C. Bhamu wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
Dear Sir<br>
For your first two points, I convinced but below suggestions triggered<br>
me to ask/clarify my doubts more:<br>
<br>
<br>
Note that depending on what you want to compare in your series of<br>
calculations it probably is a good idea to do all calculations in<br>
the same space group - even if you are left with very low symmetry.<br>
Do this by giving all Cr in the pristine cell you want to substitute<br>
a different index (Cr1, Cr2 ...).<br>
<br>
<br>
I tried to label all Cl with a number and then replaced all Cl one by<br>
one with Br. But when I initialised the structure it gives me 1-P1 SG<br>
with reduced basis vectors and all angles =60deg.<br>
I wounder how to keep all six structure in the same space group?<br>
<br>
I just had an discussion with experimentlist and he suggested me that<br>
changing of basis vector and SG is normal but the key point I have to<br>
keep in mind that the SG should not leave the the SG family belonging to<br>
that class of (here perovskite) crystal structure. For example: we have<br>
various SGs for perovskite family (225, 42, 44, 107..., etc.) and after<br>
replacing any number of Cl with Br should stabilise in one the SG<br>
belongs to perovskite structure.<br>
<br>
<br>
<br>
And remember that you do not calculate an (random) alloy, you<br>
calculate a periodic structure. Watch out for superstructure<br>
effects, perhaps by checking larger supercells with the same<br>
Br-concentration but different configuration.<br>
<br>
<br>
Sir, I am doing for bulk only like ABCl_(1-x)Br_x.<br>
<br>
Regards<br>
<br>
Bhamu<br>
<br>
<br>
Martin Pieper<br>
<br>
<br>
Am 09.05.2017 01:55, schrieb Dr. K. C. Bhamu:<br>
<br>
Dear Wien2k Users<br>
<br>
I am alloying a quaternery compound: replacing Cr with Br one by<br>
one.<br>
What I see is, the pristine compound is cubic with 225 SG. When I<br>
replace Cl by Br one by one then lattice parameters deviates from<br>
cubic to different structure.<br>
like a=b, c or a,b,c. SG also changes.<br>
During initialisation I accepted the new structure suggested by the<br>
w2web. Should I accept the suggested structure?<br>
As per my understanding from previous posts from mailing list we<br>
should accept the suggested structure which is with lower symmetry.<br>
<br>
Is the change of lattice parameters is normal behaviour of<br>
material or<br>
I am doing any mistake?<br>
<br>
Sincerely<br>
<br>
Bhamu<br>
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