# [gmx-users] SR Coulomb and LR Coulomb

Erik Lindahl lindahl at sbc.su.se
Thu Mar 30 18:36:58 CEST 2006

```Hi Pascal,

Hard to describe in words - you can find all the equations and some
graphs in chapter 4 in the manual.

Cheers,

Erik

On Mar 30, 2006, at 6:05 PM, pascal.baillod at epfl.ch wrote:

> Hi,
>
> Thank you very much, Erik and Hector, for the explanation on
> Coulomb SR and LR
> terms (copy here below). Just to get the correct picture:
>
> 1) How fast will coul_SR decrease from r=0 to r=rcoulomb? What kind
> of decrease
> function do we have there?
>
> 2) Dose coul_LR slowly increase in that interval, compensating the
> coul_SR decrease?
>
> 3) How do things work with Generalized Reaction Field for long range
> electrostatics? Is there a gradual decrease of coul_SR as in 1),
> increase of the reaction field term, or a discontinuous switch?
>
> Thank you very much!
>
> Pascal
>
>
>
>
> On Mar 30, 2006, at 1:41 PM, Hector Martinez-Seara Monne wrote:
>
>> Dear Gromacs users,
>>
>> I have what maybe is a silly question. I'm Using PME to perform my
>> calculations in bilayers and I have a set rcoulomb=1.0. What is the
>> SR coulomb and LR Coulomb energy in the log file? Can I say that LR
>> is the coulomb energy contribution of PME and SR is coulomb energy
>> contribution due to coulomb pairs interactions inside rcoulomb?
>
> Almost - SR is the direct space coulomb interaction. This is not the
> same as the full interaction inside the cutoff, since the PME
> algorithm gradually switches off direct space interaction from r=0 to
> the cutoff, and treats the rest in reciprocal space.
>
> Cheers,
>
> Erik
>
> **********************************************************************
> *********
> Pascal Baillod (PhD student)
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