"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

:line

kspace_style command :h3

[Syntax:]

kspace_style style value :pre

style = {none} or {ewald} or {pppm} or {pppm/tip4p} or {ewald/n} :ulb,l
  {none} value = none
  {ewald} value = precision
    precision = desired accuracy
  {pppm} value = precision
    precision = desired accuracy
  {pppm/tip4p} value = precision
    precision = desired accuracy
  {ewald/n} value = precision
    precision = desired accuracy :pre
:ule

[Examples:]

kspace_style pppm 1.0e-4
kspace_style none :pre

[Description:]

Define a K-space solver for LAMMPS to use each timestep to compute
long-range Coulombic interactions or long-range 1/r^N interactions.
When such a solver is used in conjunction with an appropriate pair
style, the cutoff for Coulombic or other 1/r^N interactions is
effectively infinite; each charge in the system interacts with charges
in an infinite array of periodic images of the simulation domain.

The {ewald} style performs a standard Ewald summation as described in
any solid-state physics text.

The {pppm} style invokes a particle-particle particle-mesh solver
"(Hockney)"_#Hockney which maps atom charge to a 3d mesh, uses 3d FFTs
to solve Poisson's equation on the mesh, then interpolates electric
fields on the mesh points back to the atoms.  It is closely related to
the particle-mesh Ewald technique (PME) "(Darden)"_#Darden used in
AMBER and CHARMM.  The cost of traditional Ewald summation scales as
N^(3/2) where N is the number of atoms in the system.  The PPPM solver
scales as Nlog(N) due to the FFTs, so it is almost always a faster
choice "(Pollock)"_#Pollock.

The {pppm/tip4p} style is identical to the {pppm} style except that it
adds a charge at the massless 4th site in each TIP4P water molecule.
It should be used with "pair styles"_pair_style.html with a
{long/tip4p} in their style name.

The {ewald/n} style augments {ewald} by adding long-range dispersion
sum capabilities for 1/r^N potentials and is useful for simulation of
interfaces "(Veld)"_#Veld.  It also performs standard coulombic Ewald
summations, but in a more efficient manner than the {ewald} style.
The 1/r^N capability means that Lennard-Jones or Buckingham potentials
can be used with {ewald/n} without a cutoff, i.e. they become full
long-range potentials.

Currently, only the {ewald/n} style can be used with non-orthogonal
(triclinic symmetry) simulation boxes.

When a kspace style is used, a pair style that includes the
short-range correction to the pairwise Coulombic or other 1/r^N forces
must also be selected.  For Coulombic interactions, these styles are
ones that have a {coul/long} in their style name.  For 1/r^6
dispersion forces in a Lennard-Jones or Buckingham potential, see the
"pair_style lj/coul"_pair_lj_coul.html or "pair_style
buck/coul"_pair_buck_coul.html commands.

A precision value of 1.0e-4 means one part in 10000.  This setting is
used in conjunction with the pairwise cutoff to determine the number
of K-space vectors for style {ewald} or the FFT grid size for style
{pppm}.

[Restrictions:]

A simulation must be 3d and periodic in all dimensions to use an Ewald
or PPPM solver.  The only exception is if the slab option is set with
"kspace_modify"_kspace_modify.html, in which case the xy dimensions
must be periodic and the z dimension must be non-periodic.

Kspace styles are part of the "kspace" package.  They are only enabled
if LAMMPS was built with that package.  See the "Making
LAMMPS"_Section_start.html#2_3 section for more info.

The {ewald/n} style is part of the "user-ewaldn" package.  It is only
enabled if LAMMPS was built with that package.  See the "Making
LAMMPS"_Section_start.html#2_3 section for more info.

When using a long-range pairwise TIP4P potential, you must use kspace
style {pppm/tip4p} and vice versa.

[Related commands:]

"kspace_modify"_kspace_modify.html, "pair_style"_pair_style.html
lj/cut/coul/long, "pair_style"_pair_style.html lj/charmm/coul/long,
"pair_style"_pair_style lj/coul, "pair_style"_pair_style buck/coul

[Default:]

kspace_style none :pre

:line

:link(Darden)
[(Darden)] Darden, York, Pedersen, J Chem Phys, 98, 10089 (1993).

:link(Hockney) 
[(Hockney)] Hockney and Eastwood, Computer Simulation Using Particles,
Adam Hilger, NY (1989).

:link(Pollock)
[(Pollock)] Pollock and Glosli, Comp Phys Comm, 95, 93 (1996).

:link(Veld)
[(Veld)] In 't Veld, Ismail, Grest, J Chem Phys, in press (2007).