[Pw_forum] Problem in vc-relax, radial FFT

[Ali ALLAM]

Dear all
I want to perform a vc-relax calculation for NbSi2 :
First i use the following input : 

‎&CONTROLtitle = 'NbSi2',calculation = 'vc-relax',nstep = 100,etot_conv_thr = 1.d-7,forc_conv_thr = 1.d-5,
tstress = .true.,
prefix = 'NbSi2',
verbosity = 'high',
disk_io = 'low',
tprnfor = .true.,
/
&SYSTEM
celldm(1) = 1.d0,
ibrav = 0, 
nat = 9,
ntyp = 2,
ecutwfc = 30.d0,
ecutrho = 300.d0,
nspin = 1,
occupations = 'smearing',degauss = 0.005d0,
/
&ELECTRONS
conv_thr = 1.d-7,
mixing_beta = 0.1d0,
diagonalization = 'cg',
/
&IONS
ion_dynamics = 'bfgs',
bfgs_ndim = 1,
/
&CELL
cell_dynamics = 'bfgs',
press = 0.d0,
press_conv_thr = 0.200d0,
cell_dofree = 'xyz',
/
ATOMIC_SPECIES
Si 28.0855d0 Si.pbe-n-van.UPF
Nb 92.90638d0 Nb.pbe-nsp-van.UPF

K_POINTS automatic
6 6 1 0 0 0

ATOMIC_POSITIONS crystal
Nb 0.50000 0.00000 0.00000 
Si 0.16578 0.33156 0.00000
Nb 0.00000 0.50000 0.66667 
Si 0.66844 0.83422 0.66667 
Nb 0.50000 0.50000 0.33333 
Si 0.16578 0.83422 0.33333 
Si 0.83422 0.66844 0.00000 
Si 0.33156 0.16578 0.66667 
Si 0.83422 0.16578 0.33333 

CELL_PARAMETERS
9.053677211    0.000000000    0.000000000
0.000000000    9.053677211    0.000000000
0.000000000    0.000000000  12.449514813


During convergence, the error was : Not enough space allocated for radial FFT: try restarting with a larger cell_factor
So, i use a cell_factor = 2 , then the structure was converged, but the cell parameters of the relaxed structure differs so much from the cell-parameters of the begining.
relaxed cell parameters :10.94171211    0.000000000    0.0000000000.000000000    10.94171211    0.0000000000.000000000    0.000000000   9.245414256
I try to decrease ecutwfc for 25 Ry and ecutrho = 200 Ry, and to change the 'smearing' , but always i converge the structure with a big difference between cell_parameters.the difference between cell-parameters is about 20%.
So my question is : What is the real problem in my input ?
Thanks a lot for your helping 
best regards
Ali ALLAMPhysics of materials - 2nd year PhDAix-Marseille UniversityMarseille 13013FRANCE
 		 	   		  
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