On Sun, Jun 24, 2018 at 1:17 PM, Lorenzo Monacelli < [email protected]> 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] > > > _______________________________________________ > users mailing > [email protected]https://lists.quantum-espresso.org/mailman/listinfo/users > > > > _______________________________________________ > users mailing list > [email protected] > https://lists.quantum-espresso.org/mailman/listinfo/users > -- 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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