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<p>Question:</p>
<p>You mentioned the BO as being a limitation on the accuracy of
lattice constants. Can you cite a reference on this for reading? I
was unaware that this was ever meaningfully a source of error in
an equilibrium geometric property.<br>
</p>
<p>IUPUI</p>
<div class="moz-cite-prefix">On 1/7/19 10:54 PM, Stefano Baroni
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:57F495A0-BBFF-4BB7-96F3-FED8BE8312B3@sissa.it">
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Aleksandra:
<div class=""><br class="">
</div>
<div class="">you are very right and I commend you for your
thoroughness. Only figures that can be reproduced should be
reported. Usually, lattice constants can be computed and
reported with a higher accuracy than you observe. This being
said, as Nicola says, the *numerical* accuracy that can be
achieved is usually higher than needed for practical purposes
and higher than the accuracy due to “unavoidable” physical
approximations (first and foremost the XC functional, but also
more fundamental ones, such as Born-Oppenheimer’s).
<div class=""><br class="">
</div>
<div class="">Coming to the point, that numerical results may
depend on such seemingly irrelevant “details" as hardware,
mathematical libraries, optimisation level, etc. is not
uncommon, as surprising as it may seem, particularly on
parallel machines. The result of many “commutative” operations
(sums, products) depends in practice on the order in which
these operations are performed, due to the non-commutativity
fo rounding errors, and this order does depend on all those
“details”. Some properties may be more sensitive to these
details than others in some systems more than in others. For
instance, structural parameters (such as lattice parameters or
atomic positions) are more sensitive to the accuracy in
stresses and forces in soft systems than in hard ones. Can you
tell why?</div>
<div class=""><br class="">
</div>
<div class="">Bottom line. Please, do the tests that Nicola
suggests and make sure you understand the results you get (I
am sure you are able to appreciate if and when you understand,
which is not so common). In case of need, revert to us.</div>
<div class=""><br class="">
</div>
<div class="">Good luck!</div>
<div class="">Stefano Baroni<br class="">
<div><br class="">
<blockquote type="cite" class="">
<div class="">On 8 Jan 2019, at 00:49, Aleksandra
Oranskaia <<a
href="mailto:aleksandra.oranskaia@kaust.edu.sa"
class="" moz-do-not-send="true">aleksandra.oranskaia@kaust.edu.sa</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
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<div class="">Thank you for the respond!</div>
<div class=""><br class="">
</div>
<div class="">I was confused with the difference,
because if the 3rd digit is “arbitrary” (depends on
libraries, compilers, hardware, etc.) than there is
no sense in reporting lattice constants in papers
with this accuracy; however, it is a common
practice.</div>
<div class=""><br class="">
</div>
<div class="">I used:</div>
<div class="">conv_thr=1.d-8
<div class="">tot_conv_thr=1.d-6</div>
<div class="">forc_conv_thr=1.d-6</div>
<div class="">cell_dofree=‘volume’ (for the cubic
cell)</div>
<div class=""><br class="">
</div>
<div class="">I will try to increase conv_thr and
press_conv_thr, as well as to compare gnu and
intel compillations, and reply back.</div>
<div class=""><br class="">
</div>
</div>
</div>
<br class="" style="caret-color: rgb(0, 0, 0);
font-family: Helvetica; font-size: 14px; font-style:
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<div class="" style="caret-color: rgb(0, 0, 0);
font-family: Helvetica; font-size: 14px; font-style:
normal; font-variant-caps: normal; font-weight:
normal; letter-spacing: normal; text-align: start;
text-indent: 0px; text-transform: none; white-space:
normal; word-spacing: 0px; -webkit-text-stroke-width:
0px; text-decoration: none;">
<div class="" style="word-wrap: break-word;
-webkit-nbsp-mode: space; line-break:
after-white-space;">
<div style="font-family: Helvetica; font-size: 12px;
font-style: normal; font-variant-caps: normal;
font-weight: normal; letter-spacing: normal;
text-align: start; text-indent: 0px;
text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width:
0px;" class="">Best,<br class="">
Alex.<br class="">
<br class="">
___<br class="">
Aleksandra Oranskaia (M.Sc.)<br class="">
ChemS PhD student, KAUST<br class="">
Phone: +966 50 1335254</div>
</div>
</div>
</div>
</blockquote>
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class="">
<div style="orphans: 2; widows: 2; word-wrap:
break-word; -webkit-nbsp-mode: space;
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class="Apple-style-span" style="border-collapse:
separate; border-spacing: 0px;"><span
class="Apple-style-span"
style="border-collapse: separate;
border-spacing: 0px;">
<div style="word-wrap: break-word;
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class=""><span class="Apple-style-span"
style="border-collapse: separate;
text-align: -webkit-auto; border-spacing:
0px;"><span class="Apple-style-span"
style="border-collapse: separate;
border-spacing: 0px;">
<div style="word-wrap: break-word;
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<div style="margin: 0px;" class="">
<div style="margin: 0px;" class=""><font
class="Apple-style-span"><span
style="font-size: 12px;"
class="">— </span></font></div>
<div style="color: rgb(0, 0, 0);
font-family: Helvetica; font-size:
14px; font-style: normal;
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normal; word-spacing: 0px;
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margin: 0px;" class=""><span
class="Apple-style-span"
style="font-size: 12px;">Stefano
Baroni - </span><span
style="font-size: 12px;"
class=""> SISSA, Trieste - </span><span
class="Apple-style-span"
style="font-size: 12px;"><a
href="http://stefano.baroni.me/"
class=""
moz-do-not-send="true">http://stefano.baroni.me</a>, </span><span
style="font-size: 12px;"
class="">stefanobaroni (Skype) </span></div>
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margin: 0px;" class=""><font
class="Apple-style-span"
size="3"><span
class="Apple-style-span"
style="font-size: 12px;"><br
class="">
</span></font></div>
</div>
</div>
</span></span></div>
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background-color: rgba(255, 255, 255, 0);
font-size: 12px;" class="">And aren’t we all
tired of those who claim to know the answer to
life, death and the creation being so […]
sensitive about their knowledge? [Stephen Fry]</span>
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style="background-color: rgba(255, 255, 255,
0);" class=""><br class="">
</span></div>
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<pre class="moz-quote-pre" wrap="">_______________________________________________
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<pre class="moz-signature" cols="72">--
Dr. Robert Molt Jr.
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