[Pw_forum] Problems with vc-relaxed calculation

[WANG Wei]

Dear QE users,

Why are the total energy and press of the final step of vc-relax
calculation different from those of the scf calculation using the same
structure?  Especially for the values of press, what makes this great
difference between the final step of vc-relax calculation and the final scf
calculation (after the vc-relax calculation)?

(1) For the final step of vc-relax calculation, the press and total energy
are:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
!    total energy              =     -73.74310183 Ry

          total   stress  (Ry/bohr**3)                   (kbar)     P=
-0.03
  -0.00000009   0.00000001   0.00000000         -0.01      0.00      0.00
   0.00000001  -0.00000008   0.00000000          0.00     -0.01      0.00
   0.00000000   0.00000000  -0.00000048          0.00      0.00     -0.07
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

(2) For the final scf calculations, the press and total energy are:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
!    total energy              =     -74.60031750 Ry

          total   stress  (Ry/bohr**3)                   (kbar)     P=
638.94
   0.00760001  -0.00000151   0.00000000       1118.00     -0.22      0.00
  -0.00000151   0.00759826   0.00000000         -0.22   1117.74      0.00
   0.00000000   0.00000000  -0.00216804          0.00      0.00   -318.93
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Regards,
WANG
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