[Pw_forum] Graphene structure

[Ettore Baldini Neto]

Dear QE users,

I'm kinda new with quantum espresso. I'm trying to relax a Graphene structure with  32C atoms plus one Ti atom at a given position and the structure do not converge.
In what follows I copy the input. Could anyone give-me some hint? Is there anything wrong or that can be improved? I'm using the pp's from the QE data base. I'm planning to generate my own pp's soon.
I'm using a k-point grid of 6x6x1 since with SIESTA (dzp basis) the energy and force convergence is achieved without problem.

With best regards,

************************************************************
Dr. Ettore Baldini-Neto
Wernher von Braun Center for Advanced Research
Campinas, Brazil.
************************************************************

&CONTROL
  title = 'Graphene mono layer'
  calculation =   "relax"   ,
  restart_mode = 'from_scratch' ,
  outdir='/home/usuarios/baldini/espresso-4.2.1/tmp/' ,
  pseudo_dir = '/home/usuarios/baldini/espresso-4.2.1/pseudo' ,
              etot_conv_thr = 1.0E-4  ,
              forc_conv_thr = 1.0D-3 ,
              nstep=50,
              dt=40,
              prefix='1MLG_Ti_H2',
/
&SYSTEM
                      ibrav = 0.,
                      ntyp = 2 ,
                      nat = 33 ,
                      ecutwfc = 50.0 ,
                      ecutrho = 400.0,
/
&ELECTRONS
            electron_maxstep = 300.,
            conv_thr  =  1.0d-8  ,
            mixing_mode = 'plain',
            mixing_beta = 0.5,
            diagonalization   =   'cg' ,
            diago_cg_maxiter = 50,
/
&IONS
/
CELL_PARAMETERS
16.12131954   9.383047808   0.0000000
0.00000000  18.658162370   0.0000000
0.0000000    0.0000000    56.6893424
ATOMIC_SPECIES
C   12.0107   C.pbe-van_ak.UPF
Ti  47.8670   Ti.pbe-sp-van_ak.UPF
ATOMIC_POSITIONS angstrom
C      2.89163        7.40219        1.8862
C      3.59292        8.63832        1.8826
C      3.60349        6.16702        1.8884
C      5.0186         8.64501        1.8612
C      5.0215         6.16598        1.8904
C      5.73302        7.40522        1.8718
C      1.47184        7.39616        1.891
C      2.89112        9.87066        1.886
C      2.89471        4.93264        1.8916
C      5.73319        9.8885         1.8612
C      5.73643        4.9413         1.8924
C      7.16348        7.41203        1.882
C      1.47474        4.9269         1.8924
C      0.762281       8.62783        1.8946
C      0.763134       6.16172        1.8914
C      1.4715         9.86335        1.8934
C      3.60341        11.1095        1.8868
C      5.02124        11.1239        1.882
C      3.60512        3.70156        1.8932
C      5.02645        3.70789        1.891
C      7.87807        8.65532        1.8816
C      7.16373        9.89497        1.872
C      7.16203        4.94814        1.8954
C      7.87619        6.17677        1.8996
C      5.73634        12.3522        1.8872
C      0.772689       3.69102        1.898
C      0.762793       1.22003        1.8968
C      7.87645        11.1337        1.8894
C      7.16203        12.3587        1.889
C      7.87201        13.5922        1.8942
C      2.89479        2.46619        1.8922
C      1.47482        2.45878        1.8952
Ti     6.41604        8.65974        3.74
K_POINTS {automatic} 
6.0 6.0 1.0 1.0 1.0 1.0