[Pw_forum] The resulte of optimizing the ZnO lattice constant use vc-relax

[wangqj1]

 Dear pwscf users
      I run vc-relax to  optimize the lattice constant of ZnO unit cell,my input file and out file as follows:
      
 &CONTROL
                       title = ZnOVC ,
                 calculation = 'vc-relax' ,
                restart_mode = 'from_scratch' ,
                  pseudo_dir = '/root/espresso-3.2.3/pseudo/' ,
                      prefix = ZnO ,
               etot_conv_thr = 1.D-5 ,
                       nstep = 100 ,
                     tstress = .true. ,
                     tprnfor = .true. ,
                          dt = 100 ,
                     tefield = .false. ,
 /
 &SYSTEM
                       ibrav = 4,
                   celldm(1) = 6.14161,
                   celldm(3) = 1.6022,
                         nat = 4,
                        ntyp = 2,
                     ecutwfc = 40 ,
                     ecutrho = 300 ,
                       nosym = .true. ,
                        nbnd = 25,
                 occupations = 'smearing' ,
                     degauss = 0.002 ,
                    smearing = 'methfessel-paxton' ,
                  lda_plus_u = .false. ,
 /
 &ELECTRONS
                    conv_thr = 1.D-8 ,
                 startingpot = 'atomic' ,
                 startingwfc = 'atomic' ,
                 mixing_mode = 'plain' ,
                 mixing_beta = 0.6 ,
             diagonalization = 'david' ,
              diago_full_acc = .true. ,
 /
 &IONS
                ion_dynamics = 'damp' ,
 /
 &CELL
               cell_dynamics = 'damp-w' ,
 /
ATOMIC_SPECIES
   Zn   65.00000  Zn.pz-van_ak.UPF 
    O   16.00000  O.pz-rrkjus.UPF 
ATOMIC_POSITIONS crystal 
   Zn      0.333333333    0.666666667    0.000000000    1  1  1 
    O      0.333333333    0.666666667    0.381700000    1  1  1 
   Zn      0.666666667    0.333333333    0.500000000    1  1  1 
    O      0.666666667    0.333333300    0.881700000    1  1  1 
K_POINTS automatic 
  4 4 4   0 0 0 
 
--------------------------------------------------------------------------------
  Program PWSCF     v.3.2.2  starts ...
     Today is 18Aug2008 at 23: 7:36 
     Ultrasoft (Vanderbilt) Pseudopotentials
     Current dimensions of program pwscf are:
     ntypx = 10   npk = 40000  lmax =  3
     nchix =  6  ndmx =  2000  nbrx = 14  nqfx =  8
     Title: 
     ZnOVC                                                                      

     bravais-lattice index     =            4
     lattice parameter (a_0)   =       6.1416  a.u.
     unit-cell volume          =     321.4357 (a.u.)^3
     number of atoms/cell      =            4
     number of atomic types    =            2
     kinetic-energy cutoff     =      40.0000  Ry
     charge density cutoff     =     300.0000  Ry
     convergence threshold     =      1.0E-08
     beta                      =       0.6000
     number of iterations used =            8  plain     mixing
     Exchange-correlation      =  SLA  PW   PBE  PBE (1434)
     nstep                     =          100
     celldm(1)=   6.141610  celldm(2)=   0.000000  celldm(3)=   1.602200
     celldm(4)=   0.000000  celldm(5)=   0.000000  celldm(6)=   0.000000
     crystal axes: (cart. coord. in units of a_0)
               a(1) = (  1.000000  0.000000  0.000000 )  
               a(2) = ( -0.500000  0.866025  0.000000 )  
               a(3) = (  0.000000  0.000000  1.602200 )  
     reciprocal axes: (cart. coord. in units 2 pi/a_0)
               b(1) = (  1.000000  0.577350  0.000000 )  
               b(2) = (  0.000000  1.154701  0.000000 )  
               b(3) = (  0.000000  0.000000  0.624142 )  

.......
.......
outfile:

     Final estimate of lattice vectors (input alat units)
   1.011331020   0.000000000   0.000000000
  -0.505665507   0.875838356   0.000000001
   0.000000000  -0.000000002   1.627761723
  final unit-cell volume =    334.0064 (a.u.)^3
  input alat =       6.1416 (a.u.)
CELL_PARAMETERS (alat)
   1.011331020   0.000000000   0.000000000
  -0.505665507   0.875838356   0.000000001
   0.000000000  -0.000000002   1.627761723
ATOMIC_POSITIONS (crystal)
Zn       0.333333347   0.666666677   0.000438189
O        0.333333356   0.666666677   0.380132292
Zn       0.666666690   0.333333345   0.500438960
O        0.666666695   0.333333331   0.880132483
 
from the outfile I can see the c/a=1.627761723 and from the final unit-cell volume =334.0064 (a.u.)^3,I can compute the a=6.189678(Bohr)=3.2754(A)。From the value of caculating ,we can see that the c/a and a are both bigger than the experiment , I don't konw whether the result is right or wrong .
 
Anybody who know about the question and give me the useful advice will be appreciated .
 
Sincerely 
 

 
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