charge restrained MD

[Miłosz Wieczór]

Hello cp2k users,

I'm trying to set up a charge restrained QM/MM MD simulation for a pair of 
QM molecules embedded in TIP3P water. The two molecules can undergo charge 
transfer from a (neutral + radical) to (anion radical + cation) state, and 
ideally I would like to be able to force the spin density to localize on 
one chosen molecule rather than on the other. I imagined that if I restrain 
the charge of the first molecule to -1, it will become an anion radical 
instead of a neutral molecule.

Anyway, I ran into trouble with technical details. The topic is poorly 
documented so far, so I did some tests with BECKE_RESTRAINT, 
DDAPC_RESTRAINT and MULLIKEN_RESTRAINT, and arrived at following results:

With BECKE_RESTRAINT, I could obtain the desired spin density distribution 
and produce the .cube file, but the program crashed with a segfault and no 
specific error before the first MD step was done (after the charge 
populations, densities on regular grids and decoupling energy are reported).

With DDAPC_RESTRAINT, the restraint didn't seem to do anything to the 
system, although MD could proceed with no problem (same parameters as with 
BECKE, only the type of restraint changed).

With MULLIKEN_RESTRAINT, the Mulliken populations look as desired and the 
MD goes on, but when I visualize the .cube files, I don't see any effect of 
the restraint imposed (the density resides entirely on the second molecule) 
-- which can be an artifact of the specific population analysis method.

Could anyone explain to me the differences between these approaches, and 
what can cause the crashes with BECKE? Also, should I use 
BECKE/DDAPC_RESTRAINT_A/B in conjunction with BECKE/DDAPC_RESTRAINT, or are 
they mutually exclusive? I will be grateful for any piece of advise.

In case there are any incompatibilities in the parameters, I attached the 
input file I used for BECKE_RESTRAINT. In other approaches, everything 
except for the restraint secton remained the same.

Kind regards,
-Miłosz
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