[Pw_forum] Relax structure after getting the cell relaxation result

[loc duong ding]

Dear all,

I relax the cell of structure. After that, I use
this result to relax ion positions. The input file for cell relaxation
and ion relax is the same (only change 'vc-relax' to 'relax'). These 
are the result I got.

For Vc-relax:

          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.02
   0.00000094  -0.00000009  -0.00000014          0.14     -0.01     -0.02
  -0.00000009   0.00000082  -0.00000024         -0.01      0.12     -0.04
  -0.00000014  -0.00000024  -0.00000142         -0.02     -0.04     -0.21


     bfgs converged in  62 scf cycles and  52 bfgs steps

     End of BFGS Geometry Optimization

     Final enthalpy =    -441.4525306199 Ry

CELL_PARAMETERS (alat)
   0.984130693   0.001683023   0.030237080
  -0.490603832   0.848326583  -0.013643812
   0.066169871   0.004483599   2.350435692
  

For ion relax:

          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.28
   0.00000250  -0.00000002  -0.00000113          0.37      0.00     -0.17
  -0.00000002   0.00000191  -0.00000041          0.00      0.28     -0.06
  -0.00000113  -0.00000041   0.00000128         -0.17     -0.06      0.19


     BFGS Geometry Optimization

     bfgs converged in   1 scf cycles and   0 bfgs steps

     End of BFGS Geometry Optimization

     Final energy   =    -441.4515856032 Ry

CELL_PARAMETERS (alat)
   0.984130693   0.001683023   0.030237080
  -0.490603832   0.848326583  -0.013643812
   0.066169871   0.004483599   2.350435692


I don't understand why the pressures in two case are different. I also checked the position of ions and nothing is different. 

And
another thing is Final energy and Final enthalpy. I know enthalpy is
the energy of system at constant pressure. What is difference  between
Final energy and Final enthalpy for this case?


Best regards,    
 -----------------------------------------------
Loc Duong Dinh
Ms-Ph.D Student
Sungkyunkwan Advanced Institute of Nanotechnology,
Sungkyunkwan University,
Suwon, 440-746, Korea 
Email: mambom1902 at yahoo.com