Syntax:
kspace_style style value
none value = none ewald value = precision precision = desired accuracy pppm value = precision precision = desired accuracy pppm/tip4p value = precision precision = desired accuracy
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 a standard 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).
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.
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 ones that have a coul/long in their style name.
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.
When using a pairwise TIP4P potential, you must use kspace style pppm/tip4p and vice versa.
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).