[Pw_forum] optimize monoclinic cell

[程迎春]

Dear Pwscf users,
        I did a variable cell calculations for a base centered monoclinic
crystal.  The cell displayed in xcrysden is correct, and PWscf can find the
symmetry.  However, in the optimizing process, the cell parameter changed
unexpectedly, which may be wrong.  In the user's guide of PWscf, the base
centered monoclinic cell is defined as follows:
base centered monoclinic
=============================
   v1 = (  a/2,         0,                -c/2),
   v2 = (b*cos(gamma), b*sin(gamma), 0),
   v3 = (  a/2,         0,                  c/2),
where gamma is the angle between axis a and b
=============================
In the first step of optimize process, the cell is changed as following:
CELL_PARAMETERS (alat)
   0.483414893   0.003687739  -0.271981740
  -1.013180906   0.556415406   0.000000000
   0.483414893   0.003687739   0.271981740
My question is : why the quantities of second row of v1, v3 vector do not
retain zero!

Any suggestion and comment is appriciated. Thank you in advance!

The input file is attached!
 &CONTROL
                       title = 'c2',
                      prefix = 'li',
                 calculation = 'vc-relax' ,
                restart_mode = 'from_scratch' ,
                      outdir = './' ,
                      wfcdir = './' ,
                  pseudo_dir = './' ,
                     tstress = .true. ,
                     tprnfor = .true. ,
               etot_conv_thr = 1.0D-5,
               forc_conv_thr = 1.0D-4,
 /
 &SYSTEM
                       ibrav = 13,
                   celldm(1) = 14.7756,
                   celldm(2) = 1.20528,
                   celldm(3) = 0.56348,
                   celldm(4) = -0.873177,
                         nat = 12,
                        ntyp = 1,
                     ecutwfc = 200,
                       nosym = .false. ,
 /
 &ELECTRONS
            electron_maxstep = 100,
                    conv_thr = 1.D-10,
 /
 &IONS
 ion_dynamics='bfgs',
/
 &CELL
 cell_dynamics='bfgs',
 press=900,
/
ATOMIC_SPECIES
   Li   6.914  Li.pz-s-mt.UPF
ATOMIC_POSITIONS (crystal)
Li   -0.46080   0.21740   1.49640
Li   -0.02770   0.02590   1.28350
Li   -0.80760   0.63690   0.91840
Li    0.23010   0.43900   0.29270
Li    0.06870   0.87600   0.40410
Li   -0.04760   0.70260   1.56000
Li   -1.49640  -0.21740   0.46080
Li   -1.28350  -0.02590   0.02770
Li   -0.91840  -0.63690   0.80760
Li   -0.29270  -0.43900  -0.23010
Li   -0.40410  -0.87600  -0.06870
Li   -1.56000  -0.70260   0.04760
K_POINTS automatic
  8 8 8   1 1 1
-- 
Y. C. Cheng
Department of Phyics
Nanjing University
Nanjing 210093
P. R. China
Tel: 86-25-83592907
Email: yccheng.nju at gmail.com
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