[gmx-developers] Reaction field questions

[David van der Spoel]

On 2015-03-04 00:12, Noel Carrascal wrote:
> hi again,
>
> I am interested in the development of improved methods for long range
> electrostatic
> interactions, and I started to look into the code for that.
>
> When I looked into the file name corresponding to reaction field
> calculations
>
> /src/gromacs/mdlib/rf_utils.c
>
> It all seems correct and makes absolute sense except for two lines in
> the code
> that are inside the function "real RF_excl_correction," lines 133 and 134:
>
>               129>  ener += qqA*(ek*norm2(dx) - ec);
>               130>  svmul(-2*qqA*ek, dx, df);
>               131>   rvec_inc(f[i], df);
>               132>   rvec_dec(f[k], df);
>               133>   rvec_inc(fshift[ki], df);
>               134>   rvec_dec(fshift[CENTRAL], df);
>
> What is the physical significance of modifying df by fshift[ki] and
> fshift[CENTRAL]?
> I can't understand the need of modifying forces by these two terms, does
> the manual
> says anything about this? if so, I missed it. please help.
>
> Thanks
>
> Noel Carrascal,
>
>
These forces are used to compute the virial and hence the pressure.
Having said that, if you are interested in improved methods for long 
range forces, why are you looking at reaction fields?

-- 
David van der Spoel, Ph.D., Professor of Biology
Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205.
spoel at xray.bmc.uu.se    http://folding.bmc.uu.se