forked from lijiext/lammps
106 lines
4.4 KiB
HTML
106 lines
4.4 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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<HR>
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<H3>shape command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>shape I x y z
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</PRE>
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<UL><LI>I = atom type (see asterisk form below)
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<LI>x = x diameter (distance units)
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<LI>y = y diameter (distance units)
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<LI>z = z diameter (distance units)
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>shape 1 1.0 1.0 1.0
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shape * 3.0 1.0 1.0
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shape 2* 3.0 1.0 1.0
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Set the shape for all atoms of one or more atom types. In LAMMPS,
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particles that have a finite size are said to have a "shape", as
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opposed to being a point mass. The shape can be spherical or
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aspherical, depending on whether the 3 shape values are the same or
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different. Shape values can also be set in the
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<A HREF = "read_data.html">read_data</A> data file using the "Shapes" keyword. See
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the <A HREF = "units.html">units</A> command for what distance units to use.
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</P>
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<P>The I index can be specified in one of two ways. An explicit numeric
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value can be used, as in the 1st example above. Or a wild-card
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asterisk can be used to set the shape for multiple atom types. This
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takes the form "*" or "*n" or "n*" or "m*n". If N = the number of
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atom types, then an asterisk with no numeric values means all types
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from 1 to N. A leading asterisk means all types from 1 to n
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(inclusive). A trailing asterisk means all types from n to N
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(inclusive). A middle asterisk means all types from m to n
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(inclusive).
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</P>
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<P>A line in a <A HREF = "read_data.html">data file</A> that follows the "Shapes"
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keyword specifies shape using the same format as the arguments of the
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shape command in an input script, except that no wild-card asterisk
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can be used. For example, under the "Shapes" section of a data file,
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the line that corresponds to the 1st example above would be listed as
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</P>
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<PRE>1 1.0 1.0 1.0
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</PRE>
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<P>The shape values can be set to all 0.0, which means that atoms of that
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type are point particles and not finite-size particles. Some pair
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styles and fixes and computes that operate on finite-size particles
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allow for a mixture of finite-size and point particles. See the doc
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pages of individual commands for details.
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</P>
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<P>Note that the shape command can only be used if the <A HREF = "atom_style.html">atom
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style</A> requires per-type atom shape to be set.
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Currently, only the <I>dipole</I> and <I>ellipsoid</I> styles do. The
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<I>granular</I> and <I>peri</I> styles require the shape to be set for indivual
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particles, not types. For these styles, the only option currently
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allowed is for spherical particles, so a single diameter value
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suffices to determine the shape. Per-atom diameters are defined in
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the data file read by the <A HREF = "read_data.html">read_data</A> command, or set
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to default values by the <A HREF = "create_atoms.html">create_atoms</A> command, or
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set to new values by the <A HREF = "set.html">set diamter</A> command.
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</P>
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<P>Dipoles use the atom shape to compute a moment of inertia for
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rotational energy. See the <A HREF = "pair_dipole.html">pair_style dipole</A>
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command. Only the 1st component of the shape is used since the
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particles are assumed to be spherical.
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</P>
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<P>Ellipsoids use the atom shape to compute a generalized inertia tensor.
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For example, a shape setting of 3.0 1.0 1.0 defines a particle 3x
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longer in x than in y or z and with a circular cross-section in yz.
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Degenerate ellipsoids which are spherical can be defined by setting
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all 3 shape components the same.
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</P>
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<P>If you define a <A HREF = "atom_style.html">hybrid atom style</A> which includes one
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(or more) sub-styles which require per-type shape and one (or more)
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sub-styles which require per-atom diameter, then you must define both.
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In this case the per-type shape will be ignored; only the per-atom
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diameter will be used by LAMMPS. Note that this means you can not
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currently mix aspherical particles with per-atom diameter particles.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>This command must come after the simulation box is defined by a
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<A HREF = "read_data.html">read_data</A>, <A HREF = "read_restart.html">read_restart</A>, or
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<A HREF = "create_box.html">create_box</A> command.
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</P>
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<P>All shapes must be defined before a simulation is run (if the atom
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style requires shapes be set).
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</P>
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<P><B>Related commands:</B> none
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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