Hi, guys
who can tell me what kind of correction method is used in Lammps for finite cutoff distance?
none - unless you specify it - e.g. you can use pair modify tail to
get LJ tail correction
if you like. Also pair_modify offset.
Also some of the pair styles like lj/smooth do a smoothing
at the cutoff.
Steve
Does LAMMPS support damping shift force method (Gezelter’s method) to calculate the electrostatics ?
Thanks.
Guozhen
I don't know what it is, so probabliy not. It is
an alternative to PPPM or Ewald. Is so, what
is the difference?
Steve
According to the DL_POLY2 manual, Gezelter’s method is an Damped Shift Force Coulomb sum which truncates the 1/r potential at rcut, adds a linear term to the potential in order to make both the energy and the force zero at the cutoff and furthermore introduce an additional “damping” function to moderate the 1/r dependence. It was eported to be a viable alternative to the Ewald summation.
(http://jcp.aip.org/resource/1/jcpsa6/v124/i23/p234104_s1)
Guozhen
According to the DL_POLY2 manual, Gezelter's method is an Damped Shift Force
Coulomb sum which truncates the 1/r potential at rcut, adds a linear term to
the potential in order to make both the energy and the force zero at the
cutoff and furthermore introduce an additional "damping" function to
moderate the 1/r dependence. It was eported to be a viable alternative to
the Ewald summation.
(http://jcp.aip.org/resource/1/jcpsa6/v124/i23/p234104_s1)
what you describe sounds similar to the damped coulomb
in either CHARMM or GROMACS. there also is the (generalized)
reaction field method (i have a half-ready implementation of that
somewhere on my laptop) that follows similar assumptions.
it certainly is not a viable alternative to ewald summation in a general
context. particularly, if you have an inhomogeneous distribution of the
charge density (e.g. at a phase boundary) in your system, using a
shifted force method will incur a spurious potential along the inhomogeneity.
even a 3d-ewald can lead to (smaller) problems in extreme cases
and a 2d-ewald sum might be required.
the test systems in all publications on "approximate" long-range
electrostatic methods that i have seen over the last ~15 years all
use systems that are - expectedly - most likely to show a minimal
impact. so i would strongly recommend to do careful testing before
applying any of those methods for systems, that are not as forgiving.
i would love to see anything of them use a "hard" problem for a change.
i know from colleagues that do simulations, e.g., on lipids, that even
for lipids with zwitterionic head groups, you would need either a small
ewald or pppm correction or a massive increase of the coulomb cutoff
(with damping) to get the correct behavior for the pure lipid in water.
once you use ionic lipids with counterions, you _have_ to use proper
treatment of long-range electrostatic (i.e. ewald, PPPM, (S)PME, or FMM)
or else your results will be wrong.
cheers,
axel.