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<p>Dear Chin</p>
<p>Your question is a little aside from the WIENLIST. Indeed, it is
thermodynamics. <br>
</p>
<p>In equation 1, you evaluate the enthalpie of formation of the
compound ABO3 from the reactants AO and BO2 using internal
energies. Indeed, you neglect the volumetric effect (P deltaV).
From your DFT calculation, if correct, you can simply conclude
that the following reaction is endothermic. <br>
</p>
<p>Equation 2 gives you a different reaction and in your case is
exothermic. We cannot answer in more details based on your message
which do not provide any details. I can simply say that depending
on the experimental conditions, the atmosphere can be oxygen rich
/ oxygen poor ... you can start from pure solids A and B ... or
other compounds ... In all case you will estimate a total energy
difference which will give you an idea of the endo/exo-thermic
nature of the reaction. You will not estimate the Gibbs energy
(because you also neglect the entropic part T deltaS). However, in
many articles we consider that estimation of internal energies are
sufficient for solids because both deltaV and deltaS are
negligeable (but such an approximation can be really bad). <br>
</p>
<p>Hope it will help you. <br>
</p>
<p>Cheers</p>
<p>Xavier <br>
</p>
<p><br>
</p>
<p><br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">Le 22/12/2017 à 10:32, chin Sabsu a
écrit :<br>
</div>
<blockquote
cite="mid:1512409908.3168452.1513935140461@mail.yahoo.com"
type="cite">
<div style="font-family:Helvetica Neue, Helvetica, Arial,
sans-serif;font-size:16px;">
<div>Dear Sir,</div>
<div><br>
</div>
<div>I am working on the system <g class="gr_ gr_45 gr-alert
gr_gramm gr_inline_cards gr_run_anim Style multiReplace"
id="45" data-gr-id="45">ABO3 .</g> </div>
<div><br>
</div>
<div>I have calculated formation energy in two ways: <br>
</div>
<div><br>
</div>
<div>Delta_E= Et[ABO3] - Et[AO] - Et[BO2] ------ (1)<br>
</div>
<div><br>
</div>
<div> Delta_E= Et[ABO3] - Et[A] - Et[B] - 3/2Et[O2] ----- (2)</div>
<div>
<div><br>
</div>
<div>The Delta_E is positive 0.04 Ry from Eq (1) while is
negative from Eq 2. </div>
<div><br>
</div>
<div>Why is the energy difference calculated from Eq (1)
positive?</div>
<div><br>
</div>
<div><br>
</div>
</div>
<div>Thanks in advance!</div>
<div><br>
</div>
<div><br>
</div>
<div>Regards</div>
<div><br>
</div>
<div>Chin</div>
<div><br>
</div>
</div>
<br>
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<br>
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