[QE-users] Enthalpy report in vc-relax runs

Paolo Giannozzi p.giannozzi at gmail.com
Sun Jun 24 16:57:58 CEST 2018


On Sun, Jun 24, 2018 at 1:17 PM, Lorenzo Monacelli <
lorenzo.monacelli at roma1.infn.it> wrote:

The final enthalpy is not computed with the same wavefunction as the final
> energy.
>

in this sentence and in the following, "wavefunction" must be replaced with
"plane waves"

> This is because the number of wavefunction is chosen by the original
> cutoff according to the original volume, and kept fixed during the whole
> relax.
>
> 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.
>
> 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).
>
> 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.
>
> 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).
>
> Bests,
>
> LM
>
> On 24/06/2018 12:04, Print S. wrote:
>
> Dear users,
>
> 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
>
> ...
> !    total energy              =   -2893.96255976 Ry
> ...
>
>           total   stress  (Ry/bohr**3)                   (kbar)     P=
> 5.50
> ...
>
>      Final enthalpy =   -2893.4953806269 Ry
> Begin final coordinates
>      new unit-cell volume =  12495.34754 a.u.^3 (  1851.61945 Ang^3 )
>
> ...
>
> So I got the final enthalpy as reported by pw.x (call this H_reported) and
> I calculate the enthalpy from total energy using
>
> H = E+PV
> (called H_calc)
>
>
> It turned out that H_reported and H_calc are not equal.
>
> H_reported = -2893.4953806269 Ry = -39367.97664 eV
> H_calc = E+PV=-39368.05688 eV
>
> H_reported - H_calc = -0.08024 eV
>
> So my questions are,
>
> 1) how does the pw.x determine final enthalpy in its report?
> 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?)
> 3) If both the H_reported and H_calc are theoretically correct, why do the
> values differ?
>
> 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.
>
> Thank you,
> Vichawan Sakulsupich
>
> MSc, Chulalongkorn University, THAILAND
>
>
> --
> Vichawan Sakulsupich [Print]
>
>
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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