[Pw_forum] Avoiding vacuum and obtaining a periodic relaxed structure

[Naseem Hassan]

Dear All

I have made a structure similar to propylene using chemcraft. Then I picked
the Cartesian coordinates and made an input file for relaxation. But I am
not sure How long a Cell vector I should select along z-axis (along the
Carbon chain). I want this structure to be periodic along the chain only. I
selected 9 and 10 Angstrom of lattice vector along Z-axis and obtained C-C
distance 5 and 6 Angstrom (Approx) respectively. It seems with 10 Angstrom
a little vacuum is added and C-C bonds are elongated more. if I give small
lattice vector (say 7 Ang) then C-C bond shrinks.
*How to obtain an optimized C-C bond length ? What is proper method of
avoiding vacuum and obtaining a perfect periodic relaxed structure. *
Kindly have a look at the input file.

  &control
    calculation='relax'
    restart_mode='from_scratch'
    pseudo_dir = '/share/QE_pseudo'
    outdir='./'
    prefix='Graphane'
    forc_conv_thr=1.0D-4
    nstep=3000
/
 &system
    ibrav=  0
    nat=  24,  ntyp= 2
    ecutwfc =60.0
    occupations='smearing'
    degauss=0.002
 /
 &electrons
    scf_must_converge = .false.
    mixing_mode='plain'
 /
 &ions
    ion_dynamics = 'damp'
!   ion_temperature='not_controlled'
 /
CELL_PARAMETERS {angstrom}
  10.00000000000   0.0000000000000000   0.000
  0.0000000000   10.000000   0.000
  0.0000000000000000   0.0000000000000000  10.0
ATOMIC_SPECIES
H  1.0079 H.pw-mt_fhi.UPF
C  12.0111 C.pw-mt_fhi.UPF
ATOMIC_POSITIONS {angstrom}
C        5.367107970   4.716603635   1.053018481
C        6.255850131   5.148617707   2.197431688
C        5.604070962   5.956723221   3.300642187
C        5.221138074   5.110436404   4.489234599
C        4.929412403   5.933432549   5.718863005
C        4.751718300   5.084706368   6.956077606
C        5.325054303   5.689729332   8.206876709
C        4.097184203   4.164227727   4.168731293
H        6.706474032   4.257638763   2.659887322
H        4.344006686   3.515258124   3.319860376
H        3.854647824   3.519241858   5.021187742
H        3.190910142   4.727029043   3.904250319
H        3.696251345   4.818294626   7.110426161
H        4.348967601   4.517546557   1.417990955
H        5.731392495   3.757052353   0.655098847
H        7.101013257   5.709772742   1.774202810
H        6.288921269   6.743142271   3.648137411
H        4.714167633   6.481058689   2.916034182
H        6.110659580   4.502596625   4.738962908
H        4.050783951   6.573507244   5.541152210
H        5.775952152   6.619568616   5.878615758
H        4.786163229   6.607591885   8.486624196
H        6.356503452   6.007847752   7.983988564
H        5.270145007   4.125317908   6.789870671
K_POINTS {automatic}
  3 3 1 0 0 0
Many thanks
Naseem
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