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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> or <I>rotate</I>
<PRE> <I>move</I> args = delx dely delz
delx,dely,delz = distance to displace in each dimension (distance units)
any of delx,dely,delz can be a variable (see below)
<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 (positive integer)
<I>rotate</I> args = Px Py Pz Rx Ry Rz theta
Px,Py,Pz = origin point of axis of rotation (distance units)
Rx,Ry,Rz = axis of rotation vector
theta = angle of rotation (degrees)
</PRE>
<LI>zero or more keyword/value pairs may be appended
<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
displacement vector. Any of the 3 quantities defining the vector
components can be specified as an equal-style or atom-style
<A HREF = "variable.html">variable</A>. If the value is a variable, it should be
specified as v_name, where name is the variable name. In this case,
the variable will be evaluated, and its value(s) used for the
displacement(s). The scale factor implied by the <I>units</I> keyword will
also be applied to the variable result.
</P>
<P>Equal-style variables can specify formulas with various mathematical
functions, and include <A HREF = "thermo_style.html">thermo_style</A> command
keywords for the simulation box parameters and timestep and elapsed
time. Atom-style variables can specify the same formulas as
equal-style variables but can also include per-atom values, such as
atom coordinates or per-atom values read from a file. Note that if
the variable references other <A HREF = "compute.html">compute</A> or <A HREF = "fix.html">fix</A>
commands, those values must be up-to-date for the current timestep.
See the "Variable Accuracy" section of the <A HREF = "variable.html">variable</A>
doc page for more details.
</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>The <I>rotate</I> style rotates each atom in the group by the angle <I>theta</I>
around a rotation axis <I>R</I> = (Rx,Ry,Rz) that goes thru a point <I>P</I> =
(Px,Py,Pz). The direction of rotation for the atoms around the
rotation axis is consistent with the right-hand rule: if your
right-hand's thumb points along <I>R</I>, then your fingers wrap around the
axis in the direction of positive theta.
</P>
<P>Distance units for displacements and the origin point of the <I>rotate</I>
style 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>IMPORTANT NOTE: 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, and any shrink-wrap boundary conditions (see
the <A HREF = "boundary.html">boundary</A> command) are enforced which may change
the box size. Other than this effect, this command does not change
the size or shape of the simulation box. See the
<A HREF = "change_box.html">change_box</A> command if that effect is desired.
</P>
<P>IMPORTANT NOTE: Atoms can be moved arbitrarily long distances by this
command. If the simulation box is non-periodic and shrink-wrapped
(see the <A HREF = "boundary.html">boundary</A> command), 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/shape changes dramatically, the
mapping of processors to the simulation box may not end up as optimal
as the initial mapping attempted to be.
</P>
<HR>
<P><B>Restrictions:</B>
</P>
<P>You cannot rotate around any rotation vector except the z-axis for a
2d simulation.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "lattice.html">lattice</A>, <A HREF = "change_box.html">change_box</A>,
<A HREF = "fix_move.html">fix_move</A>
</P>
<P><B>Default:</B>
</P>
<P>The option defaults are units = lattice.
</P>
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