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310 lines
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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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<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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<UL><LI>style = <I>none</I> or <I>ewald</I> or <I>ewald/disp</I> or <I>ewald/omp</I> or <I>pppm</I> or <I>pppm/cg</I> or <I>pppm/disp</I> or <I>pppm/tip4p</I> or <I>pppm/disp/tip4p</I> or <I>pppm/gpu</I> or <I>pppm/omp</I> or <I>pppm/cg/omp</I> or <I>pppm/tip4p/omp</I> or <I>msm</I> or <I>msm/omp</I>
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<PRE> <I>none</I> value = none
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<I>ewald</I> value = accuracy
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accuracy = desired relative error in forces
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<I>ewald/disp</I> value = accuracy
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accuracy = desired relative error in forces
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<I>ewald/omp</I> value = accuracy
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accuracy = desired relative error in forces
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<I>pppm</I> value = accuracy
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accuracy = desired relative error in forces
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<I>pppm/cg</I> value = accuracy (smallq)
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accuracy = desired relative error in forces
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smallq = cutoff for charges to be considered (optional) (charge units)
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<I>pppm/disp</I> value = accuracy
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accuracy = desired relative error in forces
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<I>pppm/tip4p</I> value = accuracy
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accuracy = desired relative error in forces
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<I>pppm/disp/tip4p</I> value = accuracy
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accuracy = desired relative error in forces
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<I>pppm/gpu</I> value = accuracy
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accuracy = desired relative error in forces
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<I>pppm/omp</I> value = accuracy
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accuracy = desired relative error in forces
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<I>pppm/cg/omp</I> value = accuracy
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accuracy = desired relative error in forces
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<I>pppm/tip4p/omp</I> value = accuracy
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accuracy = desired relative error in forces
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<I>msm</I> value = accuracy
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accuracy = desired relative error in forces
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<I>msm/omp</I> value = accuracy
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accuracy = desired relative error in forces
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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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kspace style msm 1.0e-4
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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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<P>Define a long-range solver for LAMMPS to use each timestep to compute
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long-range Coulombic interactions or long-range 1/r^6 interactions.
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Most of the long-range solvers perform their computation in K-space,
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hence the name of this command.
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</P>
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<P>When such a solver is used in conjunction with an appropriate pair
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style, the cutoff for Coulombic or 1/r^N interactions is effectively
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infinite. If the Coulombic case, this means each charge in the system
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interacts with charges in an infinite array of periodic images of the
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simulation domain.
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</P>
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<P>Note that using a long-range solver requires use of a matching <A HREF = "pair.html">pair
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style</A> to perform consistent short-range pairwise
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calculations. This means that the name of the pair style contains a
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matching keyword to the name of the KSpace style, as in this table:
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</P>
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<DIV ALIGN=center><TABLE BORDER=1 >
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<TR ALIGN="center"><TD >Pair style </TD><TD > KSpace style </TD></TR>
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<TR ALIGN="center"><TD >coul/long </TD><TD > ewald or pppm</TD></TR>
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<TR ALIGN="center"><TD >coul/msm </TD><TD > msm</TD></TR>
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<TR ALIGN="center"><TD >lj/long or buck/long </TD><TD > disp (for dispersion)</TD></TR>
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<TR ALIGN="center"><TD >tip4p/long </TD><TD > tip4p
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</TD></TR></TABLE></DIV>
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<HR>
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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>ewald/disp</I> style adds a long-range dispersion sum option for
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1/r^6 potentials and is useful for simulation of interfaces
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<A HREF = "#Veld">(Veld)</A>. It also performs standard Coulombic Ewald summations,
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but in a more efficient manner than the <I>ewald</I> style. The 1/r^6
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capability means that Lennard-Jones or Buckingham potentials can be
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used without a cutoff, i.e. they become full long-range potentials.
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</P>
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<P>The <I>ewald/disp</I> style can also be used with non-orthogonal (triclinic
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symmetry) simulation boxes. It is currently the only long-range
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solver that has this capability.
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</P>
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<HR>
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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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<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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<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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<HR>
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<P>The <I>pppm/disp</I> and <I>pppm/disp/tip4p</I> styles add a mesh-based long-range
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dispersion sum option for 1/r^6 potentials <A HREF = "#Isele-Holder">(Isele-Holder)</A>,
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similar to the <I>ewald/disp</I> style. The 1/r^6 capability means
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that Lennard-Jones or Buckingham potentials can be used without a cutoff,
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i.e. they become full long-range potentials.
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</P>
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<P>For these styles, it is currently recommended that you set the
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dispersion mesh size and other parameters explicitly via the
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<A HREF = "kspace_modify.html">kspace_modify</A> command, rather than let LAMMPS set
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them automatically. For example, a set of parameters that works well
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for surface systems when using real units is a LJ cutoff of 10 Angstrom,
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interpolation order = 5 (the default), grid spacing = 4.17 Angstroms,
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and Ewald parameter = 0.28. These parameters work well for the <I>ik</I>
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differentiation. For the <I>ad</I> setting, a smaller grid spacing is needed,
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e.g. 3 Angstroms. Further information on the influence of the parameters
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and how to choose them is described in <A HREF = "#Isele-Holder">(Isele-Holder)</A>.
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</P>
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<HR>
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<P>IMPORTANT NOTE: All of the PPPM styles can be used with
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single-precision FFTs by using the compiler switch -DFFT_SINGLE for
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the FFT_INC setting in your lo-level Makefile. This setting also
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changes some of the PPPM operations (e.g. mapping charge to mesh and
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interpolating electric fields to particles) to be performed in single
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precision. This option can speed-up long-range calulations,
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particularly in parallel or on GPUs. The use of the -DFFT_SINGLE flag
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is discussed in <A HREF = "Section_start.html#start_2_4">this section</A> of the
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manual. MSM does not currently support the -DFFT_SINGLE compiler switch.
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</P>
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<HR>
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<P>The <I>msm</I> style invokes a multi-level summation method MSM solver,
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<A HREF = "#Hardy">(Hardy)</A> or <A HREF = "#Hardy2">(Hardy2)</A>, which maps atom charge to a 3d
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mesh, and uses a multi-level hierarchy of coarser and coarser meshes
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on which direct coulomb solves are done. This method does not use
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FFTs and scales as N. It may therefore be faster than the other
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K-space solvers for relatively large problems when running on large
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core counts. MSM can also be used for non-periodic boundary conditions and
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for mixed periodic and non-periodic boundaries.
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</P>
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<P>MSM is most competitive versus Ewald and PPPM when only relatively
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low accuracy forces, about 1e-4 relative error or less accurate,
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are needed. Note that use of a larger coulomb cutoff (i.e. 15
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angstroms instead of 10 angstroms) provides better MSM accuracy for
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both the real space and grid computed forces.
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</P>
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<P>Currently the pressure calculation in MSM is expensive,
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so calculating the pressure at every timestep or using a fixed pressure
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simulation with MSM will cause the code to run slower.
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</P>
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<HR>
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<P>The specified <I>accuracy</I> determines the relative RMS error in per-atom
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forces calculated by the long-range solver. It is set as a
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dimensionless number, relative to the force that two unit point
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charges (e.g. 2 monovalent ions) exert on each other at a distance of
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1 Angstrom. This reference value was chosen as representative of the
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magnitude of electrostatic forces in atomic systems. Thus an accuracy
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value of 1.0e-4 means that the RMS error will be a factor of 10000
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smaller than the reference force.
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</P>
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<P>The accuracy setting is used in conjunction with the pairwise cutoff
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to determine the number of K-space vectors for style <I>ewald</I> or the
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grid size for style <I>pppm</I> or <I>msm</I>.
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</P>
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<P>RMS force errors in real space for <I>ewald</I> and <I>pppm</I> are estimated
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using equation 18 of <A HREF = "#Kolafa">(Kolafa)</A>, which is also referenced as
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equation 9 of <A HREF = "#Petersen">(Petersen)</A>. RMS force errors in K-space for
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<I>ewald</I> are estimated using equation 11 of <A HREF = "#Petersen">(Petersen)</A>,
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which is similar to equation 32 of <A HREF = "#Kolafa">(Kolafa)</A>. RMS force
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errors in K-space for <I>pppm</I> are estimated using equation 38 of
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<A HREF = "#Deserno">(Deserno)</A>. RMS force errors for <I>msm</I> are estimated
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using ideas from chapter 3 of <A HREF = "#Hardy">(Hardy)</A>, with equation 3.197
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of particular note. When using <I>msm</I> with non-periodic boundary
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conditions, it is expected that the error estimation will be too
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pessimistic.
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</P>
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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, including a <I>force</I>
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option for setting an absoulte RMS error in forces, as opposed to a
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relative RMS error.
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</P>
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<HR>
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<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> suffix are functionally
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the same as the corresponding style without the suffix. They have
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been optimized to run faster, depending on your available hardware, as
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discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A> of the
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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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<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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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, USER-OMP, and
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OPT packages respectively. They are only enabled if LAMMPS was built
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with 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">Section_accelerate</A> of the manual for
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more instructions on how to use the accelerated styles effectively.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>All of the kspace styles are part of the KSPACE package. They are
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only enabled if LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info. Note that
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the KSPACE package is installed by default.
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</P>
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<P>For MSM, a simulation must be 3d and one can use any combination of
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periodic, non-periodic, or shrink-wrapped boundaries (specified using
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the <A HREF = "boundary.html">boundary</A> command).
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</P>
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<P>For Ewald and PPPM, a simulation must be 3d and periodic in all dimensions.
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The only exception is if the slab option is set with <A HREF = "kspace_modify.html">kspace_modify</A>,
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in which case the xy dimensions must be periodic and the z dimension must be
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non-periodic.
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</P>
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<P><B>Related commands:</B>
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</P>
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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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</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 = "Deserno"></A>
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<P><B>(Deserno)</B> Deserno and Holm, J Chem Phys, 109, 7694 (1998).
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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 = "Kolafa"></A>
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<P><B>(Kolafa)</B> Kolafa and Perram, Molecular Simualtion, 9, 351 (1992).
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</P>
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<A NAME = "Petersen"></A>
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<P><B>(Petersen)</B> Petersen, J Chem Phys, 103, 3668 (1995).
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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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<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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<A NAME = "Isele-Holder"></A>
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<P><B>(Isele-Holder)</B> Isele-Holder, Mitchell, Ismail, J Chem Phys, 137, 174107 (2012).
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</P>
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<A NAME = "Hardy"></A>
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<P><B>(Hardy)</B> David Hardy thesis: Multilevel Summation for the Fast
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Evaluation of Forces for the Simulation of Biomolecules, University of
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Illinois at Urbana-Champaign, (2006).
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</P>
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<A NAME = "Hardy2"></A>
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<P><B>(Hardy)</B> Hardy, Stone, Schulten, Parallel Computing 35 (2009)
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164-177.
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</P>
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