forked from lijiext/lammps
90 lines
3.3 KiB
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90 lines
3.3 KiB
HTML
<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>
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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>gewald</I> or <I>slab</I>
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<PRE> <I>mesh</I> value = x y z
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x,y,z = PPPM FFT grid size in each dimension
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<I>order</I> value = N
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N = grid extent of Gaussian for PPPM mapping of each charge
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<I>gewald</I> value = r
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r = PPPM G-ewald parameter
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<I>slab</I> value = volfactor
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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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</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 3d FFT grid size for kspace style pppm.
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Each dimension must be factorizable into powers of 2, 3, and 5. When
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this 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 FFT grid in kspace style pppm. The
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default for this parameter is 5, which means each charge spans 5 grid
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cells in each dimension.
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</P>
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<P>The <I>gewald</I> keyword sets the value of the PPPM G-ewald parameter.
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Without this setting, LAMMPS chooses the parameter automatically as a
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function of cutoff, precision, grid spacing, etc. This means it can
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vary from one simulation to the next which may not be desirable for
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matching a KSpace solver to a pre-tabulated pairwise potential. This
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setting can also be useful if 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.
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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.
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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 = 0 0 0, order = 5, gewald = 0.0, and
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slab = 1.0.
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</P>
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