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Nicola Marzari wrote:
<blockquote cite="mid443A7E77.3000004@mit.edu" type="cite">
<blockquote type="cite">
<pre wrap="">I have a feeling (could be wrong) that this might be due to LDA-PP of
Strontium.
Since i did the same calculation for PbTiO3 using the same PPs of Ti and
O as
those in SrTiO3 and it worked well. So I appreciate if someone could
send me
your Sr LDA-PP (if you have one) that i could try.
</pre>
</blockquote>
<pre wrap=""><!---->
Hi Liping,
it's indeed baffling. Do check that your coordinates are scaled
correctly - i.e. that the atoms in cartesian coordinates are
were they should be, that the axes are what they should be.
</pre>
</blockquote>
Yes. see below:<br>
<font color="#3333ff">1x1x1:</font><br>
bravais-lattice index = 1<br>
lattice parameter (a_0) = 7.2766 a.u.<br>
unit-cell volume = 385.2939 (a.u.)^3<br>
number of atoms/cell = 5<br>
number of atomic types = 3<br>
kinetic-energy cutoff = 30.0000 Ry<br>
charge density cutoff = 270.0000 Ry<br>
convergence threshold = 1.0E-08<br>
beta = 0.7000<br>
number of iterations used = 8 plain mixing<br>
Exchange-correlation = SLA PZ NOGX NOGC (1100)<br>
celldm(1)= 7.276637 celldm(2)= 0.000000 celldm(3)= 0.000000<br>
celldm(4)= 0.000000 celldm(5)= 0.000000 celldm(6)= 0.000000<br>
<br>
crystal axes: (cart. coord. in units of a_0)<br>
a(1) = ( 1.000000 0.000000 0.000000 )<br>
a(2) = ( 0.000000 1.000000 0.000000 )<br>
a(3) = ( 0.000000 0.000000 1.000000 )<br>
......<br>
<br>
Cartesian axes<br>
<br>
site n. atom positions (a_0 units)<br>
1 Sr tau( 1) = ( 0.0000000 0.0000000
0.0000000 )<br>
2 Ti tau( 2) = ( 0.5000000 0.5000000
0.5000000 )<br>
3 O tau( 3) = ( 0.0000000 0.5000000
0.5000000 )<br>
4 O tau( 4) = ( 0.5000000 0.0000000
0.5000000 )<br>
5 O tau( 5) = ( 0.5000000 0.5000000
0.0000000 )<br>
<br>
<font color="#3333ff">2x2x1<br>
<font color="#330033"> bravais-lattice index = 6<br>
lattice parameter (a_0) = 14.5533 a.u.<br>
unit-cell volume = 1541.1754 (a.u.)^3<br>
number of atoms/cell = 20<br>
number of atomic types = 3<br>
kinetic-energy cutoff = 30.0000 Ry<br>
charge density cutoff = 270.0000 Ry<br>
convergence threshold = 1.0E-08<br>
beta = 0.7000<br>
number of iterations used = 8 plain mixing<br>
Exchange-correlation = SLA PZ NOGX NOGC (1100)<br>
celldm(1)= 14.553273 celldm(2)= 0.000000 celldm(3)= 0.500000<br>
celldm(4)= 0.000000 celldm(5)= 0.000000 celldm(6)= 0.000000<br>
<br>
crystal axes: (cart. coord. in units of a_0)<br>
a(1) = ( 1.000000 0.000000 0.000000 )<br>
a(2) = ( 0.000000 1.000000 0.000000 )<br>
a(3) = ( 0.000000 0.000000 0.500000 )<br>
...,,,<br>
<br>
Cartesian axes<br>
<br>
</font></font><font color="#330033">site n.
atom positions (a_0 units)<br>
1 Sr tau( 1) = ( 0.0000000 0.0000000
0.0000000 )<br>
2 Sr tau( 2) = ( 0.5000000 0.0000000
0.0000000 )<br>
3 Sr tau( 3) = ( 0.0000000 0.5000000
0.0000000 )<br>
4 Sr tau( 4) = ( 0.5000000 0.5000000
0.0000000 )<br>
5 O tau( 5) = ( 0.2500000 0.2500000
0.0000000 )<br>
6 O tau( 6) = ( 0.2500000 0.7500000
0.0000000 )<br>
7 O tau( 7) = ( 0.7500000 0.2500000
0.0000000 )<br>
8 O tau( 8) = ( 0.7500000 0.7500000
0.0000000 )<br>
9 Ti tau( 9) = ( 0.2500000 0.2500000
0.2500000 )<br>
10 Ti tau( 10) = ( 0.2500000 0.7500000
0.2500000 )<br>
11 Ti tau( 11) = ( 0.7500000 0.2500000
0.2500000 )<br>
12 Ti tau( 12) = ( 0.7500000 0.7500000
0.2500000 )<br>
13 O tau( 13) = ( 0.0000000 0.2500000
0.2500000 )<br>
14 O tau( 14) = ( 0.0000000 0.7500000
0.2500000 )<br>
15 O tau( 15) = ( 0.5000000 0.2500000
0.2500000 )<br>
16 O tau( 16) = ( 0.5000000 0.7500000
0.2500000 )<br>
17 O tau( 17) = ( 0.2500000 0.0000000
0.2500000 )<br>
18 O tau( 18) = ( 0.7500000 0.0000000
0.2500000 )<br>
19 O tau( 19) = ( 0.2500000 0.5000000
0.2500000 )<br>
20 O tau( 20) = ( 0.7500000 0.5000000
0.2500000 )</font><br>
<br>
<blockquote cite="mid443A7E77.3000004@mit.edu" type="cite">
<pre wrap="">
Next, if you use the same identical input files, but you put
Pb instead of Sr, do you get identical dielectric constants
from 1x1x1 and 2x2x1 ?
</pre>
</blockquote>
No, for PbTiO3, it's tetragonal structure, not cubic one as SrTiO3. The
followings are<br>
dielectric constants of PTO in tetragonal 1x1x1 unit cell and 2x2x1
supercell. <br>
Note for no reason I used 4x4x4 k-point grid for both unit cell and
super cells.<br>
<br>
1x1x1 unit cell,<br>
( 7.981121520 0.000000000 0.000000000 )<br>
( 0.000000000 7.981121520 0.000000000 )<br>
( 0.000000000 0.000000000 7.542263658 )<br>
<br>
2x2x1 supercell,<br>
( 7.775208874 0.000000000 0.000000000 )<br>
( 0.000000000 7.775208874 0.000000000 )<br>
( 0.000000000 0.000000000 7.491522972 )<br>
<br>
Maybe I should also try same k-point (4x4x4) sampling for SrTiO3 in two<br>
different cells, although they are not equivalent, and see what will
happen.<br>
<br>
Also I'll try what you suggested. Thank you very much!<br>
<br>
<blockquote cite="mid443A7E77.3000004@mit.edu" type="cite">
<pre wrap="">If you do, I admit that one would be tempted to blame the
pseudopotential, but it is still unlikely. Have a look at the
eigenvalues in the two calculations - do they match exactly ?
Let us know what you find,
nicola
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</pre>
</blockquote>
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