[CP2K:6529] Re: Only CELL_OPT with the given structure of the molecule

Florian Schiffmann flosch... at gmail.com
Tue May 5 21:17:04 UTC 2015


I haven't cheged the output file, but in general there is a problem 
defining what fixed means if you change scale your coordinate system 
(cell). The famous example of painting two dots on a ballon and blowing it 
up. The dots do not move but their distance increases as the coordinate 
systems (surface of the baloon expands). The motion of atoms you see should 
only be in absolute cartesians but not in fractional coordinates (which is 
the only meaningful way to fix them). 
If you want a given absolute distance preserved you will have to specify a 
COLVAR (e.g. distance) in TOPOLOGY, and then use it in the CONSTRAINT 
section with the RESTRAINT section set and a reasonably large K to add a 
spring term which drives your system in the desired state. 
However be careful. The Total energy now contains the contribution from the 
spring and is thus an artificial energy. If you want to compare energies in 
that case you will have to look at the SCF energies.

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