2006-09-22 00:22:34 +08:00
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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>
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<HR>
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<H3>kspace_style command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>kspace_style style value
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</PRE>
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2011-08-09 03:49:47 +08:00
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<UL><LI>style = <I>none</I> or <I>ewald</I> or <I>pppm</I> or <I>pppm/cg</I> or <I>pppm/tip4p</I> or <I>ewald/n</I> or <I>pppm/gpu</I>
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2006-09-22 00:22:34 +08:00
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<PRE> <I>none</I> value = none
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<I>ewald</I> value = precision
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precision = desired accuracy
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<I>pppm</I> value = precision
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precision = desired accuracy
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2011-08-09 03:49:47 +08:00
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<I>pppm/cg</I> value = precision (smallq)
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precision = desired accuracy
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smallq = cutoff for charges to be considered (optional) (charge units)
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<I>pppm/tip4p</I> value = precision
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2007-10-04 01:27:50 +08:00
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precision = desired accuracy
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<I>ewald/n</I> value = precision
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2011-05-02 23:01:49 +08:00
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precision = desired accuracy
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<I>pppm/gpu</I> value = precision
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precision = desired accuracy
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>kspace_style pppm 1.0e-4
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kspace_style pppm/cg 1.0e-5 1.0e-6
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2006-09-22 00:22:34 +08:00
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kspace_style none
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</PRE>
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<P><B>Description:</B>
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</P>
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2007-10-04 01:27:50 +08:00
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<P>Define a K-space solver for LAMMPS to use each timestep to compute
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long-range Coulombic interactions or long-range 1/r^N interactions.
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When such a solver is used in conjunction with an appropriate pair
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style, the cutoff for Coulombic or other 1/r^N interactions is
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effectively infinite; each charge in the system interacts with charges
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in an infinite array of periodic images of the simulation domain.
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2006-09-22 00:22:34 +08:00
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</P>
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<P>The <I>ewald</I> style performs a standard Ewald summation as described in
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any solid-state physics text.
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</P>
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<P>The <I>pppm</I> style invokes a particle-particle particle-mesh solver
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<A HREF = "#Hockney">(Hockney)</A> which maps atom charge to a 3d mesh, uses 3d FFTs
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to solve Poisson's equation on the mesh, then interpolates electric
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fields on the mesh points back to the atoms. It is closely related to
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the particle-mesh Ewald technique (PME) <A HREF = "#Darden">(Darden)</A> used in
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AMBER and CHARMM. The cost of traditional Ewald summation scales as
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N^(3/2) where N is the number of atoms in the system. The PPPM solver
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scales as Nlog(N) due to the FFTs, so it is almost always a faster
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choice <A HREF = "#Pollock">(Pollock)</A>.
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</P>
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2011-08-09 03:49:47 +08:00
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<P>The <I>pppm/cg</I> style is identical to the <I>pppm</I> style except that it
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has an optimization for systems where most particles are uncharged.
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The optional <I>smallq</I> argument defines the cutoff for the absolute
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charge value which determines whether a particle is considered charged
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or not. Its default value is 1.0e-5.
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</P>
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2006-09-22 00:22:34 +08:00
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<P>The <I>pppm/tip4p</I> style is identical to the <I>pppm</I> style except that it
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adds a charge at the massless 4th site in each TIP4P water molecule.
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It should be used with <A HREF = "pair_style.html">pair styles</A> with a
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<I>long/tip4p</I> in their style name.
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</P>
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2007-10-04 01:27:50 +08:00
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<P>The <I>ewald/n</I> style augments <I>ewald</I> by adding long-range dispersion
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sum capabilities for 1/r^N potentials and is useful for simulation of
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interfaces <A HREF = "#Veld">(Veld)</A>. It also performs standard coulombic Ewald
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summations, but in a more efficient manner than the <I>ewald</I> style.
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The 1/r^N capability means that Lennard-Jones or Buckingham potentials
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can be used with <I>ewald/n</I> without a cutoff, i.e. they become full
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long-range potentials.
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2007-10-03 06:56:45 +08:00
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</P>
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2007-10-05 22:47:55 +08:00
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<P>Currently, only the <I>ewald/n</I> style can be used with non-orthogonal
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(triclinic symmetry) simulation boxes.
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</P>
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2011-08-09 03:49:47 +08:00
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<P>Note that the PPPM styles can be used with single-precision FFTs by
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using the compiler switch -DFFT_SINGLE for the FFT_INC setting in your
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lo-level Makefile. This setting also changes some of the PPPM
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operations (e.g. mapping charge to mesh and interpolating electric
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fields to particles) to be performed in single precision. This option
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2011-08-09 04:57:05 +08:00
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can speed-up long-range calulations, particularly in parallel or on
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2011-08-26 00:46:23 +08:00
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GPUs. The use of the -DFFT_SINGLE flag is discussed in <A HREF = "Section_start.html#start_2_4">this
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section</A> of the manual.
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</P>
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2011-05-02 23:01:49 +08:00
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<HR>
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2006-09-22 00:22:34 +08:00
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<P>When a kspace style is used, a pair style that includes the
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short-range correction to the pairwise Coulombic or other 1/r^N forces
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must also be selected. For Coulombic interactions, these styles are
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2007-10-05 22:47:55 +08:00
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ones that have a <I>coul/long</I> in their style name. For 1/r^6
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dispersion forces in a Lennard-Jones or Buckingham potential, see the
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<A HREF = "pair_lj_coul.html">pair_style lj/coul</A> or <A HREF = "pair_buck_coul.html">pair_style
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buck/coul</A> commands.
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2006-09-22 00:22:34 +08:00
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</P>
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<P>A precision value of 1.0e-4 means one part in 10000. This setting is
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used in conjunction with the pairwise cutoff to determine the number
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of K-space vectors for style <I>ewald</I> or the FFT grid size for style
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<I>pppm</I>.
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</P>
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2008-08-21 06:23:13 +08:00
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<P>See the <A HREF = "kspace_modify.html">kspace_modify</A> command for additional
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options of the K-space solvers that can be set.
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</P>
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2011-05-02 23:01:49 +08:00
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<HR>
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2011-08-09 03:49:47 +08:00
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<P>Styles with a <I>cuda</I>, <I>gpu</I>, or <I>opt</I> suffix are
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2011-06-09 05:26:06 +08:00
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed in <A HREF = "Section_accelerate.html">this section</A> of
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the manual. The accelerated styles take the same arguments and should
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produce the same results, except for round-off and precision issues.
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</P>
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2011-08-09 03:49:47 +08:00
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<P>More specifically, the <I>pppm/gpu</I> style performs charge assignment and
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force interpolation calculations on the GPU. These processes are
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performed either in single or double precision, depending on whether
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the -DFFT_SINGLE setting was specified in your lo-level Makefile, as
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discussed above. The FFTs themselves are still calculated on the CPU.
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If <I>pppm/gpu</I> is used with a GPU-enabled pair style, part of the PPPM
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calculation can be performed concurrently on the GPU while other
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2011-06-09 05:26:06 +08:00
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calculations for non-bonded and bonded force calculation are performed
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on the CPU.
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</P>
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<P>These accelerated styles are part of the "user-cuda", "gpu", and "opt"
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packages respectively. They are only enabled if LAMMPS was built with
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those packages. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
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section for more info.
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</P>
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<P>See <A HREF = "Section_accelerate.html">this section</A> of the manual for more
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instructions on how to use the accelerated styles effectively.
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</P>
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2006-09-22 00:22:34 +08:00
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<P><B>Restrictions:</B>
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</P>
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<P>A simulation must be 3d and periodic in all dimensions to use an Ewald
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or PPPM solver. The only exception is if the slab option is set with
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<A HREF = "kspace_modify.html">kspace_modify</A>, in which case the xy dimensions
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must be periodic and the z dimension must be non-periodic.
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</P>
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<P>Kspace styles are part of the "kspace" package. They are only enabled
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if LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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2006-09-22 00:22:34 +08:00
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LAMMPS</A> section for more info.
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</P>
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2007-10-04 01:27:50 +08:00
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<P>The <I>ewald/n</I> style is part of the "user-ewaldn" package. It is only
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2011-08-26 00:46:23 +08:00
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enabled if LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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2007-10-04 01:27:50 +08:00
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LAMMPS</A> section for more info.
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</P>
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2006-09-22 00:22:34 +08:00
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<P>When using a long-range pairwise TIP4P potential, you must use kspace
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style <I>pppm/tip4p</I> and vice versa.
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</P>
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<P><B>Related commands:</B>
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</P>
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2010-05-07 23:11:21 +08:00
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<P><A HREF = "kspace_modify.html">kspace_modify</A>, <A HREF = "pair_lj.html">pair_style
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lj/cut/coul/long</A>, <A HREF = "pair_charmm.html">pair_style
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lj/charmm/coul/long</A>, <A HREF = "pair_lj_coul.html">pair_style
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lj/coul</A>, <A HREF = "pair_buck.html">pair_style buck/coul/long</A>
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2006-09-22 00:22:34 +08:00
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</P>
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<P><B>Default:</B>
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</P>
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<PRE>kspace_style none
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</PRE>
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<HR>
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<A NAME = "Darden"></A>
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<P><B>(Darden)</B> Darden, York, Pedersen, J Chem Phys, 98, 10089 (1993).
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</P>
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<A NAME = "Hockney"></A>
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<P><B>(Hockney)</B> Hockney and Eastwood, Computer Simulation Using Particles,
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Adam Hilger, NY (1989).
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</P>
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<A NAME = "Pollock"></A>
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<P><B>(Pollock)</B> Pollock and Glosli, Comp Phys Comm, 95, 93 (1996).
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</P>
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2007-10-04 01:27:50 +08:00
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<A NAME = "Veld"></A>
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<P><B>(Veld)</B> In 't Veld, Ismail, Grest, J Chem Phys, in press (2007).
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</P>
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</HTML>
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