forked from lijiext/lammps
99 lines
4.2 KiB
HTML
99 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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<HR>
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<H3>create_box command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>create_box N region-ID
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</PRE>
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<UL><LI>N = # of atom types to use in this simulation
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<LI>region-ID = ID of region to use as simulation domain
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>create_box 2 mybox
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>This command creates a simulation box based on the specified region.
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Thus a <A HREF = "region.html">region</A> command must first be used to define a
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geometric domain.
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</P>
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<P>The argument N is the number of atom types that will be used in the
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simulation.
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</P>
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<P>If the region is not of style <I>prism</I>, then LAMMPS encloses the region
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(block, sphere, etc) with an axis-aligned orthogonal bounding box
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which becomes the simulation domain.
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</P>
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<P>If the region is of style <I>prism</I>, LAMMPS creates a non-orthogonal
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simulation domain shaped as a parallelepiped with triclinic symmetry.
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As defined by the <A HREF = "region.html">region prism</A> command, the
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parallelepiped has its "origin" at (xlo,ylo,zlo) and is defined by 3
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edge vectors starting from the origin given by A = (xhi-xlo,0,0); B =
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(xy,yhi-ylo,0); C = (xz,yz,zhi-zlo). <I>Xy,xz,yz</I> can be 0.0 or
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positive or negative values and are called "tilt factors" because they
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are the amount of displacement applied to faces of an originally
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orthogonal box to transform it into the parallelipiped.
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</P>
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<P>A <I>prism</I> region used with the create_box command must have tilt
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factors (xy,xz,yz) that do not skew the box more than half the
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distance of the parallel box length. For example, if xlo = 2 and xhi
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= 12, then the x box length is 10 and the xy tilt factor must be
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between -5 and 5. Similarly, both xz and yz must be between
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-(xhi-xlo)/2 and +(yhi-ylo)/2. Note that this is not a limitation,
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since if the maximum tilt factor is 5 (as in this example), then
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configurations with tilt = ..., -15, -5, 5, 15, 25, ... are all
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geometrically equivalent.
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</P>
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<P>See <A HREF = "Section_howto.html#4_12">this section</A> of the doc pages for a
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geometric description of triclinic boxes, as defined by LAMMPS, and
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how to transform these parameters to and from other commonly used
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triclinic representations.
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</P>
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<P>When a prism region is used, the simulation domain must be periodic in
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any dimensions with a non-zero tilt factor, as defined by the
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<A HREF = "boundary.html">boundary</A> command. I.e. if the xy tilt factor is
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non-zero, then both the x and y dimensions must be periodic.
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Similarly, x and z must be periodic if xz is non-zero and y and z must
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be periodic if yz is non-zero. Also note that if your simulation will
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tilt the box, e.g. via the <A HREF = "fix_deform.html">fix deform</A> command, the
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simulation box must be defined as triclinic, even if the tilt factors
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are initially 0.0.
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</P>
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<P>IMPORTANT NOTE: If the system is non-periodic (in a dimension), then
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you should not make the lo/hi box dimensions (as defined in your
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<A HREF = "region.html">region</A> command) radically smaller/larger than the extent
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of the atoms you eventually plan to create, e.g. via the
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<A HREF = "create_atoms.html">create_atoms</A> command. For example, if your atoms
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extend from 0 to 50, you should not specify the box bounds as -10000
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and 10000. This is because LAMMPS uses the specified box size to
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layout the 3d grid of processors. A huge (mostly empty) box will be
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sub-optimal for performance when using "fixed" boundary conditions
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(see the <A HREF = "boundary.html">boundary</A> command). When using "shrink-wrap"
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boundary conditions (see the <A HREF = "boundary.html">boundary</A> command), a huge
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(mostly empty) box may cause a parallel simulation to lose atoms the
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first time that LAMMPS shrink-wraps the box around the atoms.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>An <A HREF = "atom_style.html">atom_style</A> and <A HREF = "region.html">region</A> must have
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been previously defined to use this command.
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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 = "create_atoms.html">create_atoms</A>, <A HREF = "region.html">region</A>
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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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