Ineffective FIXED_ATOMS constraints in QS MD

[Marco Caccin]

Dear all,

I am trying to run Quickstep MD with some fixed atoms positions (set in 
MOTION / CONSTRAINT / FIXED_ATOMS), but the positions do change over time.
Curiously enough, constrained atoms have zero velocity at frame 0 and zero 
force at every timestep, which would suggest that the constraint is somehow 
applied but ineffective.

This behaviour seems to be machine and version independent:
tested cp2k versions: 2.7 (Development Version) git: 0826abf, 2.6 
(Development Version) svn:14226, 2.5.1 svn:13632 
tested architectures/compilers: gfortran 4.8 on Darwin for cp2k 2.7, xlf 
14.1 on BlueGene/Q for cp2k 2.5 and 2.6

The attachments are a minimal example of the problem, showing 5 timesteps 
of dynamics of 3 atoms of Si of which atom 1 is (or rather, should be) 
fixed.

Is this problem known, and is there any workaround? I know there is for 
GEO_OPT runs as 
in https://groups.google.com/forum/#!topic/cp2k/KAMhME81IbM, but I haven't 
found anything applicable to MD

Kindest regards,

Marco

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