[Pw_forum] PbTiO3 Tetragonal phase optimization problem

[朱林光]

   I am a new qe user,and i have a problem about the PbTiO3 Tetragonal phase optimization using vc-relax, the output result is much less than the accurate one,i dont know how to do such optimization about tetragonal,can you give me some advice,i really appreciate it. this is my input file:
&control
calculation = 'vc-relax',
restart_mode = 'from_scratch',
pseudo_dir = '$PSEUDO_DIR/'
outdir = '$OUTPUT_DIR/'
tstress = .true.
tprnfor = .true.
disk_io='high'
/
&system
ibrav=6
celldm(1)=7.8
celldm(3)=1.04
nat=5
ntyp=3
ecutwfc=26
occupations = 'smearing'
smearing    = "gauss",
degauss=0.05,
/
&electrons
conv_thr = 1.D-8
diagonalization   =   'david'
mixing_beta=0.7D0,
/
&IONS
 /
&CELL
cell_dynamics='bfgs'
press=0.00
/
ATOMIC_SPECIES
 Pb 207.2  Pb.pz-d-van.UPF
 Ti 47.867  Ti.pz-sp-van_ak.UPF
 O 15.9994  O.pz-van_ak.UPF
ATOMIC_POSITIONS 
Pb 0.000 0.000 0.000
Ti 0.500 0.500 0.500
O 0.000 0.500 0.500
O 0.500 0.000 0.500
O 0.500 0.500 0.000
K_POINTS {automatic}
 4 4 4 1 1 1
and output result is :
Begin final coordinates
     new unit-cell volume =    401.80625 a.u.^3 (    59.54154 Ang^3 )
CELL_PARAMETERS (alat=  7.80000000)
   0.934542602   0.000000000   0.000000000
   0.000000000   0.934542602   0.000000000
   0.000000000   0.000000000   0.969470674
ATOMIC_POSITIONS (alat)
Pb       0.000000000   0.000000000   0.048659159
Ti       0.467271301   0.467271301   0.502840681
O        0.000000000   0.467271301   0.439632316
O        0.467271301   0.000000000   0.439632316
O        0.467271301   0.467271301  -0.032489462
End final coordinates
the accurate result is 3.9,and c/a is about 1.04, i dont know why my result is smaller than it? please help me,i really appreciate!