[gmx-developers] Re: water optimization?

[Berk Hess]

Erik Lindahl wrote:

> Hi,
>
>>
>> You are suggesting a seperate PME calculation for the decoupled  
>> molecule only?
>> Then you would include the interaction with all the periodic images,
>> which can have a significant contribution in vacuum.
>> In the coupled state one would also like to have all interactions,  
>> but this
>> is not possible in solution, so we use PME instead.
>> Also PME for the decoupled molecule would be much more expensive  
>> than calculating
>> plain Coulomb interactions.
>> I think explicit pairs is a much simpler solution.
>
>
> Right - I'm certainly aware of the possible complications with  
> including (or not) periodic interactions in the decoupled state, but  
> that's more of a simulation methodology rather than a technical issue  
> - if nothing else it should work fine with reaction-fields!


Yes, all of this is simulation methodology.
But I think one would always want full, non-periodic Coulomb electrostatics
for the decoupled molecule.
Reaction-field would be even worse, since there is no reaction-field in 
the vacuum
state, just vacuum. This will give a significant error in the free energy.

Berk.