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kspace_style command

Syntax:

kspace_style style value 

Examples:

kspace_style pppm 1.0e-4
kspace_style none 

Description:

Define a K-space solver for LAMMPS to use each timestep to compute long-range Coulombic interactions. When such a solver is used in conjunction with an appropriate pair style, the cutoff for Coulombic 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 an Ewald summation as described in any solid-state physics text. The pppm style invokes a particle-particle particle-mesh solver (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) 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).

When a kspace style is used, a pair style that includes the short-range correction to the pairwise Coulombic forces must also be selected. These styles are lj/cut/coul/long and lj/charmm/coul/long.

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, 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 for more info.

Related commands:

kspace_modify, pair_style lj/cut/coul/long, pair_style lj/charmm/coul/long

Default:

kspace_style none 

(Darden) Darden, York, Pedersen, J Chem Phys, 98, 10089 (1993).

(Hockney) Hockney and Eastwood, Computer Simulation Using Particles, Adam Hilger, NY (1989).

(Pollock) Pollock and Glosli, Comp Phys Comm, 95, 93 (1996).