[Pw_forum] Questions on how to define unit cell

[张 洪彬]

Hello everyone:
  I am a beginner of PWSCF and now I am tortured by the very first step 
when I want to do some calculation of mine--I really don't know how to 
define a unit cell and whether what I have create is right or wrong. In the 
structure I used in my the file below,atoms' relative location is right but 
distances between them(read form XCrySDen) are not what I want though I 
have set to, for example, celldm(1)=3.0851 angs.
  Plz someone tell me how to make it.Or take graphite as an example to show 
me how to built the unit cell. 
  Here is my input file for scf calculation of MgB2:  
&CONTROL
                 calculation = 'scf' ,
                restart_mode = 'from_scratch' ,
                      outdir = '/pwscf/tmp/' ,
                  pseudo_dir = '/pwscf/pseudo/' ,
                      prefix = 'mgb' ,
 /
 &SYSTEM
                       ibrav = 4,
                   celldm(1) = 3.0851,
                   celldm(3) = 1.14100,
                         nat = 3,
                        ntyp = 2,
                     ecutwfc = 18.0 ,
 /
 &ELECTRONS
                    conv_thr = 1.0d-8 ,
                 mixing_beta = 0.7 ,
 /
ATOMIC_SPECIES
   Mg   24.30500  Mg.pz-bhs.UPF 
    B   10.81100  B.pz-bhs.UPF 
ATOMIC_POSITIONS alat 
   Mg      0.000000000    0.000000000    0.000000000    
    B      0.500000000    0.288680000    0.570500000    
    B      0.000000000    0.577350000    0.570500000    
K_POINTS gamma


Regars

zhb

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