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

[Stefano Baroni]

I would introduce a penalty function such as A(N-C)**2, where C is the constraining coordination, and N the effective number of neighbors, calculated using your favorite definition. Not sure this has ever been implemented in QE, but it should not be a difficult task ... SB

On Jun 12, 2011, at 4:41 AM, m.g.p.krishna at chem.leidenuniv.nl wrote:

> 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
> 
> 
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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