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

[Print S.]

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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