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kspace_style.html
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
</CENTER>
<HR>
<H3>kspace_style command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>kspace_style style value
</PRE>
<UL><LI>style = <I>none</I> or <I>ewald</I> or <I>pppm</I>
<PRE> <I>none</I> value = none
<I>ewald</I> value = precision
precision = desired accuracy
<I>pppm</I> value = precision
precision = desired accuracy
<I>pppm/tip4p</I> value = precision
precision = desired accuracy
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>kspace_style pppm 1.0e-4
kspace_style none
</PRE>
<P><B>Description:</B>
</P>
<P>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.
</P>
<P>The <I>ewald</I> style performs a standard Ewald summation as described in
any solid-state physics text.
</P>
<P>The <I>pppm</I> style invokes a particle-particle particle-mesh solver
<A HREF = "#Hockney">(Hockney)</A> 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) <A HREF = "#Darden">(Darden)</A> 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 <A HREF = "#Pollock">(Pollock)</A>.
</P>
<P>The <I>pppm/tip4p</I> style is identical to the <I>pppm</I> style except that it
adds a charge at the massless 4th site in each TIP4P water molecule.
It should be used with <A HREF = "pair_style.html">pair styles</A> with a
<I>long/tip4p</I> in their style name.
</P>
<P>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 <I>coul/long</I> in their
style name.
</P>
<P>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 <I>ewald</I> or the FFT grid size for style
<I>pppm</I>.
</P>
<P><B>Restrictions:</B>
</P>
<P>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
<A HREF = "kspace_modify.html">kspace_modify</A>, in which case the xy dimensions
must be periodic and the z dimension must be non-periodic.
</P>
<P>Kspace styles are part of the "kspace" package. They are only enabled
if LAMMPS was built with that package. See the <A HREF = "Section_start.html#2_2">Making
LAMMPS</A> section for more info.
</P>
<P>When using a long-range pairwise TIP4P potential, you must use kspace
style <I>pppm/tip4p</I> and vice versa.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "kspace_modify.html">kspace_modify</A>, <A HREF = "pair_style.html">pair_style</A>
lj/cut/coul/long, <A HREF = "pair_style.html">pair_style</A> lj/charmm/coul/long
</P>
<P><B>Default:</B>
</P>
<PRE>kspace_style none
</PRE>
<HR>
<A NAME = "Darden"></A>
<P><B>(Darden)</B> Darden, York, Pedersen, J Chem Phys, 98, 10089 (1993).
</P>
<A NAME = "Hockney"></A>
<P><B>(Hockney)</B> Hockney and Eastwood, Computer Simulation Using Particles,
Adam Hilger, NY (1989).
</P>
<A NAME = "Pollock"></A>
<P><B>(Pollock)</B> Pollock and Glosli, Comp Phys Comm, 95, 93 (1996).
</P>
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