Sorry, I forgot to attach the papers. I have enclosed them to this email.
The group based cutoffs means that the entire molecule (in small organic
compounds) or some groups (with the net charge of zero) are placed inside
the cutoff. This is the basis for the group based cutoff technique in which
electrostatic forces decay faster.
so you are talking about "charge groups". they have no consequence for
ewald summation (or PPPM for that matter), since the real space
coulomb is damped smoothly to (almost) zero.
If you look into the attached papers (alternative pairwise for Ewald sum),
they basically, assumed that the electrostatic interaction is short ranged
in condensed systems and there is no Fourier space (long range). Therefore,
i know those papers, however, their assumption is only true for
homogeneous isotropic systems, which makes it useless for
most "interesting" applications.
if you have a surface, or interface that forms a surface dipole
or some higher order multipole, then those methods are wrong.
the neutralization of the charge is important inside the cutoff radius. I
think since PPPM has the long range part would not require this criteria. I
don't know how they implemented this neutralization, did they place the
entire molecule inside the cutoff radius or they use another approach. My
you have the papers, you can figure it out.
understanding is that they basically apply it through force and potential
equations. Group based cutoff (placing the entire molecule inside the cutoff
radius) is another discussion. Any comments would be appreciated.
implementing charge groups might be possible by using the "molecule"
tag in LAMMPS or you will have to write your own atomstyle that
contains the information you need. you then have to make sure that
the "skin" distance for the calculation of the neighborlists is large enough
to include you whole charge group.
In accelrys Material Studio software there are 3 options for the
electrostatic interactions, namely: 1- Ewald 2- Atom based 3- Group based
I don't know what the difference b/w Ewald and atom based is. B/c Ewald is
i assume, just truncation at cutoff. i don't care about materials studio.
doesn't it have a documentation?
essentially atom based and I don't know why they have separated it from each