[Pw_forum] monoclinic system number of atom generated is incorrect

[Lorenzo Donà]

Good morning to everyone 
I am performing this calculation on a monoclinic system: Symmetry information:
Monoclinic crystal system.
Space group number     : 15
Hall symbol            : -I 2ya
Hermann-Mauguin symbol : I12/a1
Symmetry equivalent sites:
   1         x,        y,        z
   2        -x,       -y,       -z
   3    -x+1/2,        y,       -z
   4     x+1/2,       -y,        z

Lattice parameters:
          a           b           c 
 23.8688000  11.1968000   7.8252000 
      alpha        beta       gamma 
 90.0000000  95.4750000  90.0000000 
Bravais lattice vectors :
  1.0000000   0.0000000   0.0000000 
  0.0000000   0.4690977   0.0000000 
  0.4843601   0.2345489   0.1631733
 in which the number of atom is 72 but in my pw.x output I found 80 atoms.
this is my input:

&control
    calculation='relax'
    pseudo_dir='/Users/lorenzodona/Downloads/espresso-5.4.0/pseudo/'
    verbosity= high,
/
 &system
    a= 23.8688, b= 11.1968, c= 7.8252, cosab= 0.417083157, space_group = 15,
    nat=  20, ntyp= 4,
    ecutwfc =15.0,
    rhombohedral= .false.
/
 &electrons
    diagonalization='david'
    mixing_beta = 0.7
    conv_thr= 1.0e-7
/
 &ions
 ion_dynamics='bfgs'
 /
ATOMIC_SPECIES
Zr 91.224 Zr.pbe-mt_fhi.UPF
C 12.010 C.pbe-mt_fhi.UPF
O 16.00  O.pbe-mt_fhi.UPF
H 1.00   H.pbe-mt_fhi.UPF
ATOMIC_POSITIONS crystal_sg
Zr      0.8055700   0.2260400   0.1404400
O       0.7837400   0.2764700   0.3778600
O       0.7887800   0.0352700   0.0965300
O       0.8863700   0.2930100   0.0648300
O       0.7894200   0.4193200   0.0923400
O       0.8721000   0.1295600  -0.7030000
C       0.8994700   0.1401400   0.4427600
C       0.9514400   0.0679000   0.4731700
C       0.9682300  -0.0035900   0.3401200
C       0.0168300  -0.0717400   0.3668600
C      -0.2965000  -0.2104000  -0.0790000
C      -0.7034500  -0.6656900   0.0788000
H       0.9426100  -0.0042000   0.2174500
H       0.0307000   0.8716500   0.2661000
H       0.3320300   0.3845300   0.1396500
H       0.6681300   0.8400300   0.8596200
C       0.7500000   0.4711800   0.0000000
C       0.7500000   0.9838200   0.0000000
C       0.7500000   0.8516100   0.0000000
C       0.7500000   0.6036500   0.0000000

Can you tell me where I am making a mistake??
thanks a lot 
dearly 
lorenzo
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