[gmx-developers] Reaction Filed crash!

[baptista at itqb.unl.pt]

> 3.
>> We can also discuss if, as a general rule, using a continuum reaction
>> field is better or worse than using a lattice method such as PME, but that
>> is a different issue. Like many other people (Wilfred van Gunsterem, Arieh
>> Warshel, Alan Mark, Philippe Hünenberger, etc), I'm really not convinced
>> that lattice methods offer any real advantage, because I find the
>> published evidences for either their benefits or their artifacs to be
>> rather weak or contradictory.
>
> For HOMOGENEOUS systems, then I agree; the configurations sampled can be
> achieved by cheaper methods than PME -- basically, the configurations are
> dominated by short-range effects, and beyond a certain range, all that the
> extra interactions do is affect the overall energy, which can be calculated
> pretty well by many methods (including reaction field).
>
> When lattice methods are needed is inhomogeneous systems, such as membranes
> and interfaces, because the long range order affects the configurations
> sampled, and RF and other continuum methods simply can't handle those well;
> properties will depend on cutoffs, etc.

Indeed, the symmetry of those systems is totally different and much more 
in line with a lattice model than with a a spherical isotropic RF. Still, 
the artificial periodicity along the normal to the interface may sometimes 
lead to strange effects, as in the case of a charged bilayer (charged 
lipids and/or adsorbed counterions). We know that the electric field 
should not decay as you move away from one of the monolayers (charged 
plate), because the oblique contributions from the infinite surface 
exactly cancel that decay -- a somewhat counter-intuitive effect due to 
the long-ranged order effects that you mention, and which would never be 
captured by a standard RF. However, in addition to realistically mimicking 
the infinite interface, a lattice model also places in the normal-oriented 
boxes a parallel copy of the oposite similarly charged monolayer, so that 
the field in the solvent region would cancel (two identically charged 
parallel plates), and so we lose the constant field that is so typical of 
a charged interface... So, lattice models seem a mixed blessing in this 
case. Actually, we are currently considering and testing alternative 
approaches to use with charged membranes, so any thoughts are welcome.

Best,
Antonio


-- 
Antonio M. Baptista
Instituto de Tecnologia Quimica e Biologica, Universidade Nova de Lisboa
Av. da Republica - EAN, 2780-157 Oeiras, Portugal
phone: +351-214469619         email: baptista at itqb.unl.pt
fax:   +351-214411277         WWW:   http://www.itqb.unl.pt/~baptista
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>
> Best,
> ~~~~~~~~~~~~
> Michael Shirts
> Assistant Professor
> Department of Chemical Engineering
> University of Virginia
> michael.shirts at virginia.edu
> (434)-243-1821
>
>
>
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