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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_modify 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_modify keyword value ...
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</PRE>
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<UL><LI>one or more keyword/value pairs may be listed
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<LI>keyword = <I>mesh</I> or <I>order</I> or <I>order/disp</I> or <I>overlap</I> or <I>minorder</I> or <I>force</I> or <I>gewald</I> or <I>gewald/disp</I> or <I>slab</I> or (nozforce</I> or <I>compute</I> or <I>cutoff/adjust</I> or <I>diff</I>
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<PRE> <I>mesh</I> value = x y z
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x,y,z = grid size in each dimension for long-range Coulombics
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<I>mesh/disp</I> value = x y z
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x,y,z = grid size in each dimension for 1/r^6 dispersion
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<I>order</I> value = N
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N = extent of Gaussian for PPPM or MSM mapping of charge to grid
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<I>order/disp</I> value = N
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N = extent of Gaussian for PPPM mapping of dispersion term to grid
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<I>overlap</I> = <I>yes</I> or <I>no</I> = whether the grid stencil for PPPM is allowed to overlap into more than the nearest-neighbor processor
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<I>minorder</I> value = M
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M = min allowed extent of Gaussian when auto-adjusting to minimize grid communication
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<I>force</I> value = accuracy (force units)
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<I>gewald</I> value = rinv (1/distance units)
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rinv = G-ewald parameter for Coulombics
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<I>gewald/disp</I> value = rinv (1/distance units)
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rinv = G-ewald parameter for dispersion
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<I>slab</I> value = volfactor or <I>nozforce</I>
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volfactor = ratio of the total extended volume used in the
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2d approximation compared with the volume of the simulation domain
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<I>nozforce</I> turns off kspace forces in the z direction
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<I>compute</I> value = <I>yes</I> or <I>no</I>
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<I>cutoff/adjust</I> value = <I>yes</I> or <I>no</I>
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<I>fftbench</I> value = <I>yes</I> or <I>no</I>
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<I>diff</I> value = <I>ad</I> or <I>ik</I> = 2 or 4 FFTs for PPPM in smoothed or non-smoothed mode
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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_modify mesh 24 24 30 order 6
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kspace_modify slab 3.0
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Set parameters used by the kspace solvers defined by the
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<A HREF = "kspace_style.html">kspace_style</A> command. Not all parameters are
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relevant to all kspace styles.
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</P>
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<P>The <I>mesh</I> keyword sets the grid size for kspace style <I>pppm</I> or
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<I>msm</I>. In the case of PPPM, this is the FFT mesh, and each dimension
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must be factorizable into powers of 2, 3, and 5. In the case of MSM,
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this is the finest scale real-space mesh, and each dimension must be
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factorizable into powers of 2. When this option is not set, the PPPM
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or MSM solver chooses its own grid size, consistent with the
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user-specified accuracy and pairwise cutoff. Values for x,y,z of
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0,0,0 unset the option.
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</P>
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<P>The <I>mesh/disp</I> keyword sets the grid size for kspace style
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<I>pppm/disp</I>. This is the FFT mesh for long-range dispersion and ach
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dimension must be factorizable into powers of 2, 3, and 5. When this
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option is not set, the PPPM solver chooses its own grid size,
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consistent with the user-specified accuracy and pairwise cutoff.
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Values for x,y,z of 0,0,0 unset the option.
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</P>
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<P>The <I>order</I> keyword determines how many grid spacings an atom's charge
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extends when it is mapped to the grid in kspace style <I>pppm</I> or <I>msm</I>.
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The default for this parameter is 5 for PPPM and 8 for MSM, which
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means each charge spans 5 or 4 grid cells in each dimension,
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respectively. For the LAMMPS implementation of MSM, the order can
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range from 4 to 10 and must be even. For PPPM, the minimum allowed
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setting is 2 and the maximum allowed setting is 7. The larger the
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value of this parameter, the smaller that LAMMPS will set the grid
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size, to achieve the requested accuracy. Conversely, the smaller the
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order value, the larger the grid size will be. Note that there is an
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inherent trade-off involved: a small grid will lower the cost of FFTs
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or MSM direct sum, but a larger order parameter will increase the cost
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of interpolating charge/fields to/from the grid.
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</P>
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<P>The <I>order/disp</I> keyword determines how many grid spacings an atom's
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dispersion term extends when it is mapped to the grid in kspace style
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<I>pppm/disp</I>. It has the same meaning as the <I>order</I> setting for
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Coulombics.
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</P>
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<P>The <I>overlap</I> keyword can be used in conjunction with the <I>minorder</I>
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keyword with the PPPM styles to adjust the amount of communication
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that occurs when values on the FFT grid are exchangeed between
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processors. This communication is distinct from the communication
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inherent in the parallel FFTs themselves, and is required because
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processors interpolate charge and field values using grid point values
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owned by neighboring processors (i.e. ghost point communication). If
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the <I>overlap</I> keyword is set to <I>yes</I> then this communication is
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allowed to extend beyond nearest-neighbor processors, e.g. when using
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lots of processors on a small problem. If it is set to <I>no</I> then the
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communication will be limited to nearest-neighbor processors and the
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<I>order</I> setting will be reduced if necessary, as explained by the
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<I>minorder</I> keyword discussion. The <I>overlap</I> keyword is always set to
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<I>yes</I> in MSM.
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</P>
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<P>The <I>minorder</I> keyword allows LAMMPS to reduce the <I>order</I> setting if
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necessary to keep the communication of ghost grid point limited to
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exchanges between nearest-neighbor processors. See the discussion of
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the <I>overlap</I> keyword for details. If the <I>overlap</I> keyword is set to
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<I>yes</I>, which is the default, this is never needed. If it set to <I>no</I>
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and overlap occurs, then LAMMPS will reduce the order setting, one
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step at a time, until the ghost grid overlap only extends to nearest
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neighbor processors. The <I>minorder</I> keyword limits how small the
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<I>order</I> setting can become. The minimum allowed value for PPPM is 2,
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which is the default. If <I>minorder</I> is set to the same value as
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<I>order</I> then no reduction is allowed, and LAMMPS will generate an
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error if the grid communcation is non-nearest-neighbor and <I>overlap</I>
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is set to <I>no</I>. The <I>minorder</I> keyword is not currently supported in
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MSM.
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</P>
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<P>The PPPM order parameter may be reset by LAMMPS when it sets up the
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FFT grid if the implied grid stencil extends beyond the grid cells
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owned by neighboring processors. Typically this will only occur when
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small problems are run on large numbers of processors. A warning will
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be generated indicating the order parameter is being reduced to allow
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LAMMPS to run the problem. Automatic adjustment of the order parameter
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is not supported in MSM.
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</P>
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<P>The <I>force</I> keyword overrides the relative accuracy parameter set by
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the <A HREF = "kspace_style.html">kspace_style</A> command with an absolute
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accuracy. The accuracy determines the RMS error in per-atom forces
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calculated by the long-range solver and is thus specified in force
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units. A negative value for the accuracy setting means to use the
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relative accuracy parameter. The accuracy setting is used in
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conjunction with the pairwise cutoff to determine the number of
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K-space vectors for style <I>ewald</I>, the FFT grid size for style
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<I>pppm</I>, or the real space grid size for style <I>msm</I>.
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</P>
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<P>The <I>gewald</I> keyword sets the value of the Ewald or PPPM G-ewald
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parameter for charge as <I>rinv</I> in reciprocal distance units. Without
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this setting, LAMMPS chooses the parameter automatically as a function
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of cutoff, precision, grid spacing, etc. This means it can vary from
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one simulation to the next which may not be desirable for matching a
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KSpace solver to a pre-tabulated pairwise potential. This setting can
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also be useful if Ewald or PPPM fails to choose a good grid spacing
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and G-ewald parameter automatically. If the value is set to 0.0,
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LAMMPS will choose the G-ewald parameter automatically. MSM does not
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use the <I>gewald</I> parameter.
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</P>
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<P>The <I>gewald/disp</I> keyword sets the value of the Ewald or PPPM G-ewald
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parameter for dispersion as <I>rinv</I> in reciprocal distance units. It
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has the same meaning as the <I>gewald</I> setting for Coulombics.
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</P>
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<P>The <I>slab</I> keyword allows an Ewald or PPPM solver to be used for a
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systems that are periodic in x,y but non-periodic in z - a
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<A HREF = "boundary.html">boundary</A> setting of "boundary p p f". This is done by
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treating the system as if it were periodic in z, but inserting empty
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volume between atom slabs and removing dipole inter-slab interactions
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so that slab-slab interactions are effectively turned off. The
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volfactor value sets the ratio of the extended dimension in z divided
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by the actual dimension in z. The recommended value is 3.0. A larger
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value is inefficient; a smaller value introduces unwanted slab-slab
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interactions. The use of fixed boundaries in z means that the user
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must prevent particle migration beyond the initial z-bounds, typically
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by providing a wall-style fix. The methodology behind the <I>slab</I>
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option is explained in the paper by <A HREF = "#Yeh">(Yeh)</A>. An alternative slab
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option can be invoked with the <I>nozforce</I> keyword in lieu of the
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volfactor. This turns off all kspace forces in the z direction.
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The <I>nozforce</I> option is not supported by MSM. For MSM, any combination
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of periodic, non-periodic, or shrink-wrapped boundaries can be set
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using <A HREF = "boundary.html">boundary</A> (the slab approximation in not needed).
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</P>
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<P>The <I>compute</I> keyword allows Kspace computations to be turned off,
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even though a <A HREF = "kspace_style.html">kspace_style</A> is defined. This is
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not useful for running a real simulation, but can be useful for
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debugging purposes or for computing only partial forces that do not
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include the Kspace contribution. You can also do this by simply not
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defining a <A HREF = "kspace_style.html">kspace_style</A>, but a Kspace-compatible
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<A HREF = "pair_style.html">pair_style</A> requires a kspace style to be defined.
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This keyword gives you that option.
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</P>
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<P>The <I>cutoff/adjust</I> keyword applies only to MSM. If this option is
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turned on, the Coulombic cutoff will be automatically adjusted at the
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beginning of the run to give the desired estimated error. Other
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cutoffs such as LJ will not be affected. If the grid is not set using
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the <I>mesh</I> command, this command will also attempt to use the optimal
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grid that minimizes cost using an estimate given by
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<A HREF = "#Hardy">(Hardy)</A>. Note that this cost estimate is not exact, somewhat
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experimental, and still may not yield the optimal parameters.
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</P>
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<P>The <I>fftbench</I> keyword applies only to PPPM. It is on by default. If
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this option is turned off, LAMMPS will not take the time at the end
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of a run to give FFT benchmark timings, and will finish a few seconds
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faster than it would if this option were on.
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</P>
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<P>The <I>diff</I> keyword specifies the differentiation scheme used by the
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PPPM method to compute forces on particles given electrostatic
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potentials on the PPPM mesh. The <I>ik</I> approach is the default for
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PPPM and is the original formulation used in <A HREF = "#Hockney">(Hockney)</A>. It
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performs differentiation in Kspace, and uses 3 FFTs to transfer each
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component of the computed fields back to real space for total of 4
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FFTs per timestep.
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</P>
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<P>The analytic differentiation <I>ad</I> approach uses only 1 FFT to transfer
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information back to real space for a total of 2 FFTs per timestep. It
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then performs analytic differentiation on the single quantity to
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generate the 3 components of the electric field at each grid point.
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This is sometimes referred to as "smoothed" PPPM. This approach
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requires a somewhat larger PPPM mesh to achieve the same accuracy as
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the <I>ik</I> method. The <I>ad</I> method is always used for MSM.
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</P>
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<P>IMPORTANT NOTE: Currently, not all PPPM styles support the <I>ad</I>
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option. Support for those PPPM variants will be added later.
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</P>
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<P><B>Restrictions:</B> none
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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_style.html">kspace_style</A>, <A HREF = "boundary.html">boundary</A>
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</P>
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<P><B>Default:</B>
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</P>
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<P>The option defaults are mesh = mesh/disp = 0 0 0, order = order/disp =
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5 (PPPM), order = 8 (MSM), minorder = 2, overlap = yes, force = -1.0,
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gewald = gewald/disp = 0.0, slab = 1.0, compute = yes, cutoff/adjust =
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yes (MSM), fftbench = yes (PPPM), and diff = ik (PPPM).
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</P>
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<HR>
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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 = "Yeh"></A>
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<P><B>(Yeh)</B> Yeh and Berkowitz, J Chem Phys, 111, 3155 (1999).
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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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