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rLAMMPS lammps
kspace_style.html
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<HTML>
<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> or <I>pppm/tip4p</I> or <I>ewald/n</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
<I>ewald/n</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 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.
</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>The <I>ewald/n</I> style augments <I>ewald</I> by adding long-range dispersion
sum capabilities for 1/r^N potentials and is useful for simulation of
interfaces <A HREF = "#Veld">(Veld)</A>. It also performs standard coulombic Ewald
summations, but in a more efficient manner than the <I>ewald</I> style.
The 1/r^N capability means that Lennard-Jones or Buckingham potentials
can be used with <I>ewald/n</I> without a cutoff, i.e. they become full
long-range potentials.
</P>
<P>Currently, only the <I>ewald/n</I> style can be used with non-orthogonal
(triclinic symmetry) simulation boxes.
</P>
<P>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 <I>coul/long</I> in their style name. For 1/r^6
dispersion forces in a Lennard-Jones or Buckingham potential, see the
<A HREF = "pair_lj_coul.html">pair_style lj/coul</A> or <A HREF = "pair_buck_coul.html">pair_style
buck/coul</A> commands.
</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_3">Making
LAMMPS</A> section for more info.
</P>
<P>The <I>ewald/n</I> style is part of the "user-ewaldn" package. It is only
enabled if LAMMPS was built with that package. See the <A HREF = "Section_start.html#2_3">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,
<A HREF = "pair_style">pair_style</A> lj/coul, <A HREF = "pair_style">pair_style</A> buck/coul
</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>
<A NAME = "Veld"></A>
<P><B>(Veld)</B> In 't Veld, Ismail, Grest, J Chem Phys, in press (2007).
</P>
</HTML>
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