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

[Wolfram Quester]

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

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