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<p>Hi,<br>
</p>
<p>The final enthalpy is not computed with the same wavefunction as
the final energy.</p>
<p>This is because the number of wave-function is chosen by the
original cutoff according to the original volume, and kept fixed
during the whole relax.</p>
<p>However, since you are changing the volume, the number of
wavefunction that satisfy the cutoff condition changes. This is
why at the end of the vc-relax pw automatically does an extra scf
calculation, redetermining the number of wf to use by the original
cutoff at the final volume.</p>
<p>This is why energy / forces and stress tensor of the last
calculation differs a bit from the last relaxation step (even if
the cell and the atoms are the same).</p>
<p><br>
</p>
<p>The enthalpy printed is computed in the last step of vc-relax,
while the energy you are taking is computed in the last scf
calculation.</p>
<p>Therefore the correct result of the enthalpy is the one you
performed manually adding the PV factor to the energy (note that
this difference should go to zero as the cutoff -> infinity).</p>
<p>Bests,</p>
<p>LM<br>
</p>
<br>
<div class="moz-cite-prefix">On 24/06/2018 12:04, Print S. wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAM_nK+c-D30mJiavyP9_1j1O4Ht9amxUH_HExEbHLzg0MBFdQQ@mail.gmail.com">
<div dir="ltr">
<div>Dear users,</div>
<div><br>
</div>
<div>I have done vc-relax jobs using pw.x binary v.6.0 (svn rev.
13079). It completed with this information in the final step<br>
</div>
<div><br>
</div>
<div>...<br>
! total energy = -2893.96255976 Ry<br>
</div>
<div>...</div>
<br>
<div> total stress (Ry/bohr**3)
(kbar) P= 5.50<br>
</div>
<div>...<br>
<br>
Final enthalpy = -2893.4953806269 Ry<br>
Begin final coordinates<br>
new unit-cell volume = 12495.34754 a.u.^3 ( 1851.61945
Ang^3 )<br>
</div>
<div><br>
</div>
<div>...</div>
<div><br>
</div>
<div>So I got the final enthalpy as reported by pw.x (call this
H_reported) and I calculate the enthalpy from total energy
using <br>
</div>
<div><br>
</div>
<div>H = E+PV</div>
<div>(called H_calc)<br>
</div>
<div><br>
</div>
<div><br>
</div>
<div>It turned out that H_reported and H_calc are not equal. <br>
</div>
<div><br>
</div>
<div>H_reported = -2893.4953806269 Ry = -39367.97664 eV</div>
<div>H_calc = E+PV=-39368.05688 eV<br>
</div>
<div><br>
</div>
<div>H_reported - H_calc = -0.08024 eV<br>
</div>
<div><br>
</div>
<div>So my questions are, <br>
</div>
<div><br>
</div>
<div>1) how does the pw.x determine final enthalpy in its
report? <br>
</div>
<div>2) Is it theoretically correct to determine H=E+P*V
directly from E, P and V reported in pw.x output. (Or should I
determine P(V) by fitting the equation of state to E-V curve
then derive H from that?)<br>
</div>
<div>3) If both the H_reported and H_calc are theoretically
correct, why do the values differ?</div>
<div><br>
</div>
<div>My problem is not in this single case, it is that I did
several relax jobs and the discrepancies in H_reported and
H_calc are not systematic.<br>
</div>
<div><br>
</div>
<div>Thank you,</div>
<div>Vichawan Sakulsupich</div>
<div><br>
</div>
<div>MSc, Chulalongkorn University, THAILAND<br>
</div>
<div><br>
</div>
<div><br>
-- <br>
<div class="gmail_signature">Vichawan Sakulsupich [Print]</div>
</div>
</div>
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