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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>
</CENTER>
<HR>
<H3>displace_atoms command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>displace_atoms group-ID style args keyword value ...
</PRE>
<UL><LI>group-ID = ID of group of atoms to displace
<LI>style = <I>move</I> or <I>ramp</I> or <I>random</I>
<PRE> <I>move</I> args = delx dely delz
delx,dely,delz = distance to displace in each dimension (distance units)
<I>ramp</I> args = ddim dlo dhi dim clo chi
ddim = <I>x</I> or <I>y</I> or <I>z</I>
dlo,dhi = displacement distance between dlo and dhi (distance units)
dim = <I>x</I> or <I>y</I> or <I>z</I>
clo,chi = lower and upper bound of domain to displace (distance units)
<I>random</I> args = dx dy dz seed
dx,dy,dz = random displacement magnitude in each dimension (distance units)
seed = random # seed (8 digits or less)
</PRE>
<LI>zero or more keyword/value pairs may be appended to the args
<PRE> keyword = <I>units</I>
value = <I>box</I> or <I>lattice</I>
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>displace_atoms top move 0 -5 0 units box
displace_atoms flow ramp x 0.0 5.0 y 2.0 20.5
</PRE>
<P><B>Description:</B>
</P>
<P>Displace a group of atoms. This can be used to move atoms a large
distance before beginning a simulation or to randomize atoms initially
on a lattice. For example, in a shear simulation, an initial strain
can be imposed on the system. Or two groups of atoms can be brought
into closer proximity.
</P>
<P>The <I>move</I> style displaces the group of atoms by the specified 3d
distance.
</P>
<P>The <I>ramp</I> style displaces atoms a variable amount in one dimension
depending on the atom's coordinate in a (possibly) different
dimension. For example, the second example command displaces atoms in
the x-direction an amount between 0.0 and 5.0 distance units. Each
atom's displacement depends on the fractional distance its y
coordinate is between 2.0 and 20.5. Atoms with y-coordinates outside
those bounds will be moved the minimum (0.0) or maximum (5.0) amount.
</P>
<P>The <I>random</I> style independently moves each atom in the group by a
random displacement, uniformly sampled from a value between -dx and
+dx in the x dimension, and similarly for y and z. Random numbers are
used in such a way that the displacement of a particular atom is the
same, regardless of how many processors are being used.
</P>
<P>Distance units for displacement are determined by the setting of <I>box</I>
or <I>lattice</I> for the <I>units</I> keyword. <I>Box</I> means distance units as
defined by the <A HREF = "units.html">units</A> command - e.g. Angstroms for <I>real</I>
units. <I>Lattice</I> means distance units are in lattice spacings. The
<A HREF = "lattice.html">lattice</A> command must have been previously used to
define the lattice spacing.
</P>
<HR>
<P>Care should be taken not to move atoms on top of other atoms. After
the move, atoms are remapped into the periodic simulation box if
needed.
</P>
<P>Atoms can be moved arbitrarily long distances by this command. If the
simulation box is non-periodic, this can change its size or shape.
This is not a problem, except that the mapping of processors to the
simulation box is not changed by this command from its initial 3d
configuration; see the <A HREF = "processors.html">processors</A> command. Thus, if
the box size or shape changes dramatically, the simulation may not be
as well load-balanced (atoms per processor) as the initial mapping
tried to achieve.
</P>
<P><B>Restrictions:</B>
</P>
<P>This command requires inter-processor communication to migrate atoms
once they have been displaced. This means that your system must be
ready to perform a simulation before using this command (force fields
are setup, atom masses are set, etc).
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "lattice.html">lattice</A>
</P>
<P><B>Default:</B>
</P>
<P>The option defaults are units = lattice.
</P>
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