forked from lijiext/lammps
108 lines
4.2 KiB
HTML
108 lines
4.2 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>displace_atoms command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>displace_atoms group-ID style args keyword value ...
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</PRE>
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<UL><LI>group-ID = ID of group of atoms to displace
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<LI>style = <I>move</I> or <I>ramp</I> or <I>random</I>
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<PRE> <I>move</I> args = delx dely delz
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delx,dely,delz = distance to displace in each dimension (distance units)
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<I>ramp</I> args = ddim dlo dhi dim clo chi
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ddim = <I>x</I> or <I>y</I> or <I>z</I>
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dlo,dhi = displacement distance between dlo and dhi (distance units)
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dim = <I>x</I> or <I>y</I> or <I>z</I>
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clo,chi = lower and upper bound of domain to displace (distance units)
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<I>random</I> args = dx dy dz seed
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dx,dy,dz = random displacement magnitude in each dimension (distance units)
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seed = random # seed (positive integer)
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</PRE>
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<LI>zero or more keyword/value pairs may be appended
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<PRE> keyword = <I>units</I>
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value = <I>box</I> or <I>lattice</I>
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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>displace_atoms top move 0 -5 0 units box
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displace_atoms flow ramp x 0.0 5.0 y 2.0 20.5
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Displace a group of atoms. This can be used to move atoms a large
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distance before beginning a simulation or to randomize atoms initially
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on a lattice. For example, in a shear simulation, an initial strain
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can be imposed on the system. Or two groups of atoms can be brought
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into closer proximity.
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</P>
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<P>The <I>move</I> style displaces the group of atoms by the specified 3d
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distance.
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</P>
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<P>The <I>ramp</I> style displaces atoms a variable amount in one dimension
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depending on the atom's coordinate in a (possibly) different
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dimension. For example, the second example command displaces atoms in
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the x-direction an amount between 0.0 and 5.0 distance units. Each
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atom's displacement depends on the fractional distance its y
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coordinate is between 2.0 and 20.5. Atoms with y-coordinates outside
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those bounds will be moved the minimum (0.0) or maximum (5.0) amount.
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</P>
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<P>The <I>random</I> style independently moves each atom in the group by a
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random displacement, uniformly sampled from a value between -dx and
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+dx in the x dimension, and similarly for y and z. Random numbers are
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used in such a way that the displacement of a particular atom is the
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same, regardless of how many processors are being used.
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</P>
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<P>Distance units for displacement are determined by the setting of <I>box</I>
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or <I>lattice</I> for the <I>units</I> keyword. <I>Box</I> means distance units as
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defined by the <A HREF = "units.html">units</A> command - e.g. Angstroms for <I>real</I>
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units. <I>Lattice</I> means distance units are in lattice spacings. The
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<A HREF = "lattice.html">lattice</A> command must have been previously used to
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define the lattice spacing.
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</P>
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<HR>
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<P>IMPORTANT NOTE: Care should be taken not to move atoms on top of other
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atoms. After the move, atoms are remapped into the periodic
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simulation box if needed, and any shrink-wrap boundary conditions (see
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the <A HREF = "boundary.html">boundary</A> command) are enforced which may change
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the box size. Other than this effect, this command does not change
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the size or shape of the simulation box. See the
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<A HREF = "change_box.html">change_box</A> command if that effect is desired.
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</P>
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<P>IMPORTANT NOTE: Atoms can be moved arbitrarily long distances by this
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command. If the simulation box is non-periodic and shrink-wrapped
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(see the <A HREF = "boundary.html">boundary</A> command), this can change its size
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or shape. This is not a problem, except that the mapping of
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processors to the simulation box is not changed by this command from
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its initial 3d configuration; see the <A HREF = "processors.html">processors</A>
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command. Thus, if the box size/shape changes dramatically, the
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mapping of processors to the simulation box may not end up as optimal
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as the initial mapping attempted to be.
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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 = "lattice.html">lattice</A>, <A HREF = "change_box.html">change_box</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 units = lattice.
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</P>
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</HTML>
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