forked from lijiext/lammps
192 lines
8.3 KiB
HTML
192 lines
8.3 KiB
HTML
<HTML>
|
|
<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_box command
|
|
</H3>
|
|
<P><B>Syntax:</B>
|
|
</P>
|
|
<PRE>displace_box group-ID parameter args ... keyword value ...
|
|
</PRE>
|
|
<UL><LI>group-ID = ID of group of atoms to displace
|
|
|
|
<LI>one or more parameter/arg pairs may be appended
|
|
|
|
<PRE>parameter = <I>x</I> or <I>y</I> or <I>z</I> or <I>xy</I> or <I>xz</I> or <I>yz</I>
|
|
<I>x</I>, <I>y</I>, <I>z</I> args = style value(s)
|
|
style = <I>final</I> or <I>delta</I> or <I>scale</I> or <I>volume</I>
|
|
<I>final</I> values = lo hi
|
|
lo hi = box boundaries at end of run (distance units)
|
|
<I>delta</I> values = dlo dhi
|
|
dlo dhi = change in box boundaries at end of run (distance units)
|
|
<I>scale</I> values = factor
|
|
factor = multiplicative factor for change in box length at end of run
|
|
<I>volume</I> value = none = adjust this dim to preserve volume of system
|
|
<I>xy</I>, <I>xz</I>, <I>yz</I> args = style value
|
|
style = <I>final</I> or <I>delta</I>
|
|
<I>final</I> value = tilt
|
|
tilt = tilt factor at end of run (distance units)
|
|
<I>delta</I> value = dtilt
|
|
dtilt = change in tilt factor at end of run (distance units)
|
|
</PRE>
|
|
<LI>zero or more keyword/value pairs may be appended
|
|
|
|
<LI>keyword = <I>remap</I> or <I>units</I>
|
|
|
|
<PRE> <I>remap</I> value = <I>x</I> or <I>none</I>
|
|
x = remap coords of atoms in group into deforming box
|
|
none = no remapping of coords
|
|
<I>units</I> value = <I>lattice</I> or <I>box</I>
|
|
lattice = distances are defined in lattice units
|
|
box = distances are defined in simulation box units
|
|
</PRE>
|
|
|
|
</UL>
|
|
<P><B>Examples:</B>
|
|
</P>
|
|
<PRE>displace_box all xy final -2.0 z final 0.0 5.0 units box
|
|
displace_box all x scale 1.1 y volume z volume
|
|
</PRE>
|
|
<P><B>Description:</B>
|
|
</P>
|
|
<P>Change the volume and/or shape of the simulation box. Orthogonal
|
|
simulation boxes have 3 adjustable parameters (x,y,z). Triclinic
|
|
(non-orthogonal) simulation boxes have 6 adjustable parameters
|
|
(x,y,z,xy,xz,yz). Any or all of them can be adjusted independently
|
|
and simultaneously by this command. This fix can be used to expand or
|
|
contract a box, or to apply a shear strain to a non-orthogonal box.
|
|
</P>
|
|
<P>Any parameter varied by this command must refer to a periodic
|
|
dimension - see the <A HREF = "boundary.html">boundary</A> command. For parameters
|
|
"xy", "xz", and "yz" this means both affected dimensions must be
|
|
periodic, e.g. x and y for "xy". Dimensions not varied by this
|
|
command can be periodic or non-periodic.
|
|
</P>
|
|
<P>The size and shape of the initial simulation box are specified by the
|
|
<A HREF = "create_box.html">create_box</A> or <A HREF = "read_data.html">read_data</A> or
|
|
<A HREF = "read_restart.html">read_restart</A> command used to setup the simulation,
|
|
or they are the values from the end of the previous run. The
|
|
<A HREF = "create_box.html">create_box</A>, <A HREF = "read_data.html">read data</A>, and
|
|
<A HREF = "read_restart.html">read_restart</A> commands also determine whether the
|
|
simulation box is orthogonal or triclinic and their doc pages explain
|
|
the meaning of the xy,xz,yz tilt factors. If the displace_box command
|
|
changes the xy,xz,yz tilt factors, then the simulation box must be
|
|
triclinic, even if its initial tilt factors are 0.0.
|
|
</P>
|
|
<HR>
|
|
|
|
<P>For the <I>x</I>, <I>y</I>, and <I>z</I> parameters, this is the meaning of their
|
|
styles and values.
|
|
</P>
|
|
<P>For style <I>final</I>, the final lo and hi box boundaries of a dimension
|
|
are specified. The values can be in lattice or box distance units.
|
|
See the discussion of the units keyword below.
|
|
</P>
|
|
<P>For style <I>delta</I>, plus or minus changes in the lo/hi box boundaries
|
|
of a dimension are specified. The values can be in lattice or box
|
|
distance units. See the discussion of the units keyword below.
|
|
</P>
|
|
<P>For style <I>scale</I>, a multiplicative factor to apply to the box length
|
|
of a dimension is specified. For example, if the initial box length
|
|
is 10, and the factor is 1.1, then the final box length will be 11. A
|
|
factor less than 1.0 means compression.
|
|
</P>
|
|
<P>The <I>volume</I> style changes the specified dimension in such a way that
|
|
the box volume remains constant while other box dimensions are changed
|
|
explicitly via the styles discussed above. For example, "x scale 1.1
|
|
y scale 1.1 z volume" will shrink the z box length as the x,y box
|
|
lengths increase, to keep the volume constant (product of x,y,z
|
|
lengths). If "x scale 1.1 z volume" is specified and parameter <I>y</I> is
|
|
unspecified, then the z box length will shrink as x increases to keep
|
|
the product of x,z lengths constant. If "x scale 1.1 y volume z
|
|
volume" is specified, then both the y,z box lengths will shrink as x
|
|
increases to keep the volume constant (product of x,y,z lengths). In
|
|
this case, the y,z box lengths shrink so as to keep their relative
|
|
aspect ratio constant.
|
|
</P>
|
|
<P>For solids or liquids, note that when one dimension of the box is
|
|
expanded by this command, it may be physically undesirable to hold the
|
|
other 2 box lengths constant (unspecified by this command) since that
|
|
implies a density change. Using the <I>volume</I> style for those 2
|
|
dimensions to keep the box volume constant may make more physical
|
|
sense, but may also not be correct for materials and potentials whose
|
|
Poisson ratio is not 0.5.
|
|
</P>
|
|
<P>For the <I>scale</I> and <I>volume</I> styles, the box length is expanded or
|
|
compressed around its mid point.
|
|
</P>
|
|
<HR>
|
|
|
|
<P>For the <I>xy</I>, <I>xz</I>, and <I>yz</I> parameters, this is the meaning of their
|
|
styles and values. Note that changing the tilt factors of a triclinic
|
|
box does not change its volume.
|
|
</P>
|
|
<P>For style <I>final</I>, the final tilt factor is specified. The value
|
|
can be in lattice or box distance units. See the discussion of the
|
|
units keyword below.
|
|
</P>
|
|
<P>For style <I>delta</I>, a plus or minus change in the tilt factor is
|
|
specified. The value can be in lattice or box distance units. See
|
|
the discussion of the units keyword below.
|
|
</P>
|
|
<P>All of these styles change the xy, xz, yz tilt factors. In LAMMPS,
|
|
tilt factors (xy,xz,yz) for triclinic boxes are always bounded by half
|
|
the distance of the parallel box length. For example, if xlo = 2 and
|
|
xhi = 12, then the x box length is 10 and the xy tilt factor must be
|
|
between -5 and 5. Similarly, both xz and yz must be between
|
|
-(xhi-xlo)/2 and +(yhi-ylo)/2. Note that this is not a limitation,
|
|
since if the maximum tilt factor is 5 (as in this example), then
|
|
configurations with tilt = ..., -15, -5, 5, 15, 25, ... are all
|
|
equivalent. Any tilt factor specified by this command must be within
|
|
these limits.
|
|
</P>
|
|
<HR>
|
|
|
|
<P>The <I>remap</I> keyword determines whether atom positions are re-mapped to
|
|
the new box. If <I>remap</I> is set to <I>x</I> (the default), atoms in the fix
|
|
group are re-mapped; otherwise they are not. If <I>remap</I> is set to
|
|
<I>none</I>, then this remapping does not take place.
|
|
</P>
|
|
<P>The <I>units</I> keyword determines the meaning of the distance units used
|
|
to define various arguments. A <I>box</I> value selects standard distance
|
|
units as defined by the <A HREF = "units.html">units</A> command, e.g. Angstroms for
|
|
units = real or metal. A <I>lattice</I> value means the 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>The simulation box size or shape can be changed by arbitrarily large
|
|
amounts by this command. 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>Any box dimension varied by this fix must be periodic.
|
|
</P>
|
|
<P>This command requires inter-processor communication to migrate atoms
|
|
once they have moved. 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 = "fix_deform.html">fix deform</A>
|
|
</P>
|
|
<P><B>Default:</B>
|
|
</P>
|
|
<P>The option defaults are remap = x and units = lattice.
|
|
</P>
|
|
</HTML>
|