[CPMD-list] Problems optimizing geometry using FIX STRUCTURE DIST

[Ari P Seitsonen]

Dear Wolfram,

  CPMD has sometimes problems with geometry optimisation and constraints; 
it's more typical to use molecular dynamics and usually simulated 
annealing, employing very low temperatures (and finally reaching 
practically zero).

  Some one who knows this part better can probably give a better 
explanation.

    Greetings from Zurich,

       apsi

PS It's usually advantageous to use the 'LBFGS' for geometry 
  optimisations, in case you didn't specify it already

-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-
 Ari Paavo Seitsonen / Ari.P.Seitsonen at iki.fi / http://www.iki.fi/~apsi/
 Tel +41 1 635 44 97 / Fax +41 1 635 68 38 / GSM +41 79 719 09 35      
 Anschrift: Physikalisch Chemisches Institut (PCI), Universität Zürich (UniZh)
 Indirizzo: Winterthurerstrasse 190, CH-8057 Zürich
 Address:   Schweiz / Svizzera / Suisse / Svizra / Switzerland

On Mon, 17 Jan 2005, Wolfram Quester wrote:

> Dear CPMD users and developers,
> 
> I try to calculate bonding energies of Si-clusters. To give predefined
> distances between the clusters I have in my input file
> 
> &SYSTEM
> ANGSTROM
> SYMMETRY
> 0
> POISSON SOLVER TUCKERMAN
> CELL ABSOLUTE DEGREE
> 40.0 20.0 20.0 90 90 90
> CUTOFF
> 20.0
> &END
> 
> &ATOMS
> Silicon
> [...snip...]
> CONSTRAINTS
> FIX STRUCTURE
> 1
> DIST 1 8 1.200000
> END CONSTRAINTS
> &END
> 
> From the documentation I expected that these constraints keep the
> distance between atoms 1 and 8 fixed during the simulation run. But in
> my output I have in the beginning
> 
> 
>                      <<<<< CONSTRAINTS >>>>>
>  FIXED STRUCTURE ELEMENTS
>       TYPE     ATOM ATOM ATOM ATOM                VALUE      DIFFERENCE
>   DISTANCE       1    8    0    0               2.26767   -1.776357E-15
> 
> and in the end:
> 
>                      <<<<< CONSTRAINTS >>>>>
>  FIXED STRUCTURE ELEMENTS
>       TYPE     ATOM ATOM ATOM ATOM                VALUE      DIFFERENCE
>   DISTANCE       1    8    0    0               2.92527    6.575977E-01
> 
> This means that that now the distance between the two atoms is 0.658
> Bohr larger than in the beginning. But if it is "fixed", this value
> shouldn't change? Or am I wrong? And I don't get a file CONSTRAINT
> created, which should be there.
> 
> Thanks for your hints,
> 
> Wolfi
> 
>