[Pw_forum] Constraint optimization: how to fix coordination number?

[m.g.p.krishna at chem.leidenuniv.nl]

Dear List,

In many situations, one need to constraint the .coordination. number  
of atoms, so that, say, a chemically adsorbed molecule wont move away  
from the reactant
surface (ie. to keep coordination number as a constant, bonds never  
break w.r.t. a cutoff radius).

In order to test that constraint optimization in Quantum Espresso  
[4.3], I use an ethane molecule as test case.

My doubt:
How can one constraint the .coordination number. of an atom (so that  
the bonds of that particular atom, will remain the same before and  
after the optimizatio
n, regardless of the ions displacements).

Please note that in my test.in I used a shorter C-C bond length  
(1.3Ang.) which is supposed to increase and it didn.t. I used   
'atom_coord' and  'type_coord'
  and in all time it tried to keep the bond fixed!

Can any one give an idea on how to fix the coordination number in a  
geometry optimization? Hope some one can also explain why it always  
fixes the bondlength
and not following the <0 0 0> and <0 0 1> flags?

Please also see below for my test run,

Thanks in advance,

Krishna Mohan GP
Center for Superfunctional Materials
POSTECH
Sotuh Korea.


+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

&CONTROL
                  calculation = 'relax' ,
                 restart_mode = 'from_scratch' ,
                       outdir = './' ,
                   pseudo_dir = '/home01/a202kks/krishna/myPSP' ,
                       prefix = 'Final_' ,
                    verbosity = 'high' ,
                        nstep = 50 ,
                    tprnfor   = .true.,
  /
  &SYSTEM
                        ibrav = 0,
                     celldm(1)= 1.,
                          nat = 8,
                         ntyp = 2,
                      ecutwfc = 20 ,
                        nosym = .true. ,
occupations = 'smearing',
smearing      = 'gauss',
degauss       = 0.002,
/
  &ELECTRONS
/
  &IONS
ion_dynamics='damp',
/

   CONSTRAINTS
   1 0.5
   'atom_coord' 5 1 3.5 0.5

   CELL_PARAMETERS Cubic
12 0  0
0  12 0
0  0  12
  ATOMIC_SPECIES
     C   12.0107  C.pbe-paw_kj.UPF
     H   1.00674  H.pbe-paw_kj.UPF
  K_POINTS automatic
  3 3 3 0 0 0
  ATOMIC_POSITIONS angstrom
  C                 -0.00000000   -0.00000000    0.6500000 0 0 1
  H                  0.00000000    1.00880567    1.0066666 0 0 0
  H                  0.87365134   -0.50440284    1.0066666 0 0 0
  H                 -0.87365134   -0.50440284    1.0066666 0 0 0
  C                 -0.00000000   -0.00000000   -0.6500000 0 0 1
  H                  0.87365134    0.50440284   -1.0066666 0 0 0
  H                 -0.87365134    0.50440284   -1.0066666 0 0 0
  H                 -0.00000000   -1.00880567   -1.0066666 0 0 0

-----------------------------------------------------------------------------
After geometry optimization, I Got:
      Damped Dynamics: convergence achieved in  11 steps
. . .   . . .
Begin final coordinates
      new unit-cell volume =   1728.00000 a.u.^3 (   256.06318 Ang^3 )

CELL_PARAMETERS (alat=  1.00000000)
   12.000000000   0.000000000   0.000000000
    0.000000000  12.000000000   0.000000000
    0.000000000   0.000000000  12.000000000

ATOMIC_POSITIONS (angstrom)
C        0.000000000  -0.001564190   0.649998147
H        0.000000000   1.058348951   1.147005976
H        0.919069360  -0.531092625   1.145321656
H       -0.919069360  -0.531092625   1.145321656
C        0.000000000   0.001564190  -0.649998147
H        0.919069360   0.531092625  -1.145321656
H       -0.919069360   0.531092625  -1.145321656
H        0.000000000  -1.058348951  -1.147005976
End final coordinates