[gmx-developers] Reaction Filed crash!

[baptista]

 On Sun, 18 Dec 2011 16:21:34 +0100, David van der Spoel wrote:
> On 12/18/11 12:48 AM, baptista at itqb.unl.pt wrote:
>>
>>> 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.
>
> If you have a charged membrane with sufficient counter ions (and
> salt) you have a dipolar system and the field will fall off with
> distance from the membrane, if you leave out the counter-ions it will
> remain constant. Using PBC the field will be enhanced artificially. 
> We
> have derived an analytical relation for this effect in the supporting
> information of this paper: http://dx.doi.org/10.1002/anie.200703987
>
> Your best bet is to establish using PME and counter-ions how large
> the PBC effect is, then you can probably change the epsilon_surface
> slightly to reduce the effect. The exact value should depend on the
> box size. I have not tried this myself though.

 Sorry, I should have been more clear. Naturally, the addition of ions 
 will lead to a distance-dependent field (Gouy-Chapman, etc). My point 
 was that the lattice method creates a wrong physical model to start 
 with. So, if you think of the electrostatic effect of adding the ions in 
 terms of a Debye (not Guntelberg) charging process, the initial state 
 would be completely wrong, which is not particularly encouraging... 
 Anyway, I must look at the paper you mention. Thanks also for the 
 suggested approach, which is similar to some we are considering.

 Cheers,
 Antonio

>
>>
>> Best,
>> Antonio
>>
>>
>>
>>
>
>
> --
> David.
> 
> ________________________________________________________________________
> David van der Spoel, PhD, Professor of Biology
> Dept. of Cell and Molecular Biology, Uppsala University.
> Husargatan 3, Box 596,  	75124 Uppsala, Sweden
> phone:	46 18 471 4205		fax: 46 18 511 755
> spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se
> 
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