charge restrained MD

[vincenzo verdolino]

Hi Milosz,

I know this is a quite old post but i am experiencing similar problems and 
I was wondering whether you were able to figure out or not. In my case i 
have a zeolite with two metallic centers and i want to track a chemical 
reaction which is suppose to be red-ox. In order to force the oxidation 
state at least on one of the two metallic centers I tested 
both BECKE_RESTRAINT, DDAPC_RESTRAINT. The first one brings very quickly 
the whole system to explode moving the lighter atoms first and eventually 
all the others in a kind of fully repulsive field. The second approach is 
much more stable and allows me to force a charge/spin localization on one 
of the two metallic atoms but after a while the mulliken charges have a 
discontinuity which get along to the beforementioned atomic "explosion".

Would be extremely halpfull any suggestion

best regards

Vincenzo

Il giorno mercoledì 29 ottobre 2014 23:03:52 UTC+1, Miłosz Wieczór ha 
scritto:
>
> 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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