[CPMD-list] Re: Equilibration of organic molecules in water

[Ian R. Greig]

> 
> IG> Additionally, it seems that any study of the reactivity of a small
> system will probably be practically limited to Car-Parrinello
> dynamics. This will ignore any change in volume on reaction which
> might be quite significant?
> 
> could you elaborate a little more on this. there are quite a few ways
> to determine e.g. free energy surfaces, some of which are implemented in
> cpmd. but of course any reaction that happens spontaneously and without
> any additional driving in the timeframe available to CPMD simulations,
> has to be very fast (e.g. proton transfers) and i cannot imagine how 
> something like that would result in a large change in volume.
>

Sorry for just throwing this in at the end. I was referring to a system in
which something other than a spontaneous proton tranfer or the like was
occurring. For example, a bonding change between heavy atom, which would
require use of one of the implemented means of driving the system along
the reaction coordinate of interest. 

The point I was trying to make was that such processes (consider the
oft-used symmetric displacement reaction occurring at a carbon center in
solution for example) might be more likely to involve a significant change
in volume at constant pressure along the reaction coordinate given that
computationally feasible systems will use a solute surrounded by only a
couple of solvation shells.

All the best,

Ian

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                          Ian R. Greig

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