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[Manuel Pérez Jigato <Manuel.PerezJigato@fys.kuleuven.be>]

Dear all

after computing the optimal volume/lattice constant of a rock-salt  
oxide for a fixed k-point set and planewave cut-off energy,

1- if i then carry out a total energy calculation for the optimal  
lattice constant
2- and if i start the volume/lattice constant optimisation starting  
from the same
  optimal lattice constant

in both cases i get completely different stress: please see numbers below

let me ask, is the difference explained by the absence of the cut-off  
smoothing
in the total energy calculation, and that is used during cell optimisation

thanks

Manuel Perez Jigato

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1- total energy calculation

 Cartesian components of stress tensor (hartree/bohr^3)
  sigma(1 1)=  4.90452484E-05  sigma(3 2)=  0.00000000E+00
  sigma(2 2)=  4.90452484E-05  sigma(3 1)=  0.00000000E+00
  sigma(3 3)=  4.90452484E-05  sigma(2 1)=  0.00000000E+00

-Cartesian components of stress tensor (GPa)         [Pressure=  
-1.4430E+00 GPa]
- sigma(1 1)=  1.44296084E+00  sigma(3 2)=  0.00000000E+00
- sigma(2 2)=  1.44296084E+00  sigma(3 1)=  0.00000000E+00
- sigma(3 3)=  1.44296084E+00  sigma(2 1)=  0.00000000E+00


2- cell-optimisation

 Cartesian components of stress tensor (hartree/bohr^3)
  sigma(1 1)=  1.25262834E-07  sigma(3 2)=  0.00000000E+00
  sigma(2 2)=  1.25262834E-07  sigma(3 1)=  0.00000000E+00
  sigma(3 3)=  1.25262834E-07  sigma(2 1)=  0.00000000E+00

-Cartesian components of stress tensor (GPa)         [Pressure=  
-3.6854E-03 GPa]
- sigma(1 1)=  3.68535934E-03  sigma(3 2)=  0.00000000E+00
- sigma(2 2)=  3.68535934E-03  sigma(3 1)=  0.00000000E+00
- sigma(3 3)=  3.68535934E-03  sigma(2 1)=  0.00000000E+00


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