[gmx-users] How to add another electrostatic summation methodin Gromacs

[Mark Abraham]

Shuangxing Dai wrote:
> Yes, but I still cannot write out the pair interaction for each ion 
> since there is cut off. With cut off, I need to assign a averge value of 
> the last two constants to each ion. But I cannot get the total number of 
> neighbours in a cut off distance in advance. So I cannot use the user 
> defined part to do my eletrostatic summation.

If N is the number of neighbours inside the cutoff distance, then you 
might be right. If so, then I don't understand why the inner summation 
constrains "j != i" and "r_ij < R_c" since the latter is implied by 
formation of the set over which the summations are occurring.

If N is the total number of ions then the last few terms are all 
system-wide constants.

I also don't understand why the limit expression exists. For any values 
actually used in a simulation, that expression is just a constant.

> I compared carefully with Wolf summation with the Ewald summation, the 
> differences are:
> 1. There is no reciprocal term in Wolf.
> 2.Instead, the Wolf assums that the ions out side the truncations sphere 
> is exactly located at the sphere. That is how the extra two terms come out.
> 3. Wolf uses cut-off, while Ewald can only deal with peoriodical 
> boundary conditions cases.
> So I need to add a similiar summation like Ewald to treat with the 
> long-range electrostatic force, not a non-bonded interaction. Then which 
> parts needs to be read and modified?

I don't understand your conclusion, here.

Mark