[Pw_forum] how to read lattice constant from the output file of vc-relax

[sun jason]

Dear all,

I execute a vc-relax calculation, the output file is like this:
================================================================================

     End of self-consistent calculation
 
!    total energy              =   -26.26021699 ryd
     estimated scf accuracy    <        2.4E-10 ryd
 
     convergence has been achieved
 
     Forces acting on atoms (Ry/au):
 
     atom   1 type  1   force =     0.00000000    0.00000000    0.00000000
     atom   2 type  2   force =     0.00000000    0.00000000    0.00000000
 
     Total force =     0.000000     Total SCF correction =     0.000000
 
 
     entering subroutine stress ...
 
          total   stress  (ryd/bohr**3)                  (kbar)     P=   
-0.20
  -0.00000138   0.00000000   0.00000000         -0.20      0.00      0.00
   0.00000000  -0.00000138   0.00000000          0.00     -0.20      0.00
   0.00000000   0.00000000  -0.00000138          0.00      0.00     -0.20
 
 
     Wentzcovitch Damped Dynamics: convergence achieved, Efinal=   
-26.26021699
 
------------------------------------------------------------------------
 
     Final estimate of lattice vectors (input alat units)
  -0.500982241   0.000000000   0.500982241
   0.000000000   0.500982241   0.500982241
  -0.500982241   0.500982241   0.000000000
  final unit-cell volume =     80.2290 (a.u.)^3
  input alat =       6.8330 (a.u.)
 
CELL_PARAMETERS (alat)
  -0.500982241   0.000000000   0.500982241
   0.000000000   0.500982241   0.500982241
  -0.500982241   0.500982241   0.000000000
 
ATOMIC_POSITIONS (crystal)
B        0.000000000   0.000000000   0.000000000
N        0.250000000   0.250000000   0.250000000
 
 
 
     Writing file        bn.save for program phonon
========================================================================================



does it mean that the lattice constant is changed to 
(0.500982241/0.5)*3.615=3.6221 angstrom? if it is right, the lattice 
constant optimized by GGA pseudopotential using pwscf code is large than 
the experimental value, but the GGA result by CASTEP code is small than the 
experimental value. which one is reasonable?

any help would be appreciated.

best regards,


==============================================
Jian Sun 
Physics Dept. of Nanjing University 
National Lab. of Solid State Microstructures 
22 Hankou Road, Gulou District 
Nanjing, Jiangsu Province 
210093 
China
==============================================