2006-09-22 00:22:34 +08:00
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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_atoms command
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</H3>
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<P><B>Syntax:</B>
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</P>
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2007-06-20 20:56:17 +08:00
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<PRE>create_atoms type style args keyword values ...
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2006-09-22 00:22:34 +08:00
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</PRE>
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<UL><LI>type = atom type (1-Ntypes) of atoms to create
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2007-06-20 20:56:17 +08:00
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<LI>style = <I>box</I> or <I>region</I> or <I>single</I>
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2007-06-20 20:56:17 +08:00
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<PRE> <I>box</I> args = none
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<I>region</I> args = region-ID
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region-ID = atoms will only be created if contained in the region
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<I>single</I> args = x y z
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x,y,z = coordinates of a single atom (distance units)
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</PRE>
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2007-07-03 04:04:44 +08:00
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<LI>zero or more keyword/value pairs may be appended
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<LI>keyword = <I>basis</I> or <I>units</I>
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<PRE> <I>basis</I> values = M itype
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M = which basis atom
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itype = atom type (1-N) to assign to this basis atom
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<I>units</I> value = <I>lattice</I> or <I>box</I>
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<I>lattice</I> = the geometry is defined in lattice units
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<I>box</I> = the geometry is defined in simulation box units
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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>create_atoms 1 box
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create_atoms 3 region regsphere basis 2 3
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create_atoms 3 single 0 0 5
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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 atoms on a lattice or a single atom as an
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alternative to reading in their coordinates via a
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<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
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command. A simulation box must already exist, which is typically
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created via the <A HREF = "create_box.html">create_box</A> command. Before using
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this command, a lattice must also be defined using the
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<A HREF = "lattice.html">lattice</A> command. The only exception is for the
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<I>single</I> style with units = box.
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</P>
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<P>For the <I>box</I> style, the create_atoms command fills the entire
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2007-08-07 21:59:29 +08:00
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simulation box with atoms on the lattice. If your simulation box is
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periodic, you should insure its size is a multiple of the lattice
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spacings, to avoid unwanted atom overlaps at the box boundaries. If
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your box is periodic and a multiple of the lattice spacing in a
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particular dimension, LAMMPS is careful to put exactly one atom at the
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boundary (on either side of the box), not zero or two.
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</P>
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<P>For the <I>region</I> style, the geometric volume is filled that is inside
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the simulation box and is also consistent with the region volume. See
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the <A HREF = "region.html">region</A> command for details. Note that a region can
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be specified so that its "volume" is either inside or outside a
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geometric boundary. Also note that if your region is the same size as
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a periodic simulation box (in some dimension), LAMMPS does not
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implement the same logic as with the <I>box</I> style, to insure exactly
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one atom at the boundary. if this is what you desire, you should
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either use the <I>box</I> style, or tweak the region size to get precisely
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the atoms you want.
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</P>
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<P>For the <I>single</I> style, a single atom is added to the system at the
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specified coordinates. This can be useful for debugging purposes or
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to create a tiny system with a handful of atoms at specified
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positions.
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</P>
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<P>The <I>basis</I> keyword specifies an atom type that will be assigned to
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specific basis atoms as they are created. See the
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<A HREF = "lattice.html">lattice</A> command for specifics on how basis atoms are
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defined for the unit cell of the lattice. By default, all created
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atoms are assigned the argument <I>type</I> as their atom type.
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</P>
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<P>The <I>units</I> keyword determines the meaning of the distance units used
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to specify the coordinates of the one atom created by the <I>single</I>
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style. A <I>box</I> value selects standard distance units as defined by
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the <A HREF = "units.html">units</A> command, e.g. Angstroms for units = real or
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metal. A <I>lattice</I> value means the distance units are in lattice
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spacings.
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</P>
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<P>Note that this command adds atoms to those that already exist. By
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using the create_atoms command multiple times, multiple sets of atoms
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can be added to the simulation. For example, interleaving
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create_atoms with <A HREF = "lattice.html">lattice</A> commands specifying different
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orientations, grain boundaries can be created. By using the
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create_atoms command in conjunction with the
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<A HREF = "delete_atoms.html">delete_atoms</A> command, reasonably complex
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geometries can be created. The create_atoms command can also be used
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to add atoms to a system previously read in from a data or restart
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file. In all these cases, care should be taken to insure that new
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atoms do not overlap existing atoms inappropriately. The
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<A HREF = "delete_atoms.html">delete_atoms</A> command can be used to handle
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overlaps.
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</P>
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<P>Atom IDs are assigned to created atoms in the following way. The
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collection of created atoms are assigned consecutive IDs that start
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immediately following the largest atom ID existing before the
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create_atoms command was invoked. When a simulation is performed on
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different numbers of processors, there is no guarantee a particular
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created atom will be assigned the same ID.
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</P>
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2009-01-20 01:17:01 +08:00
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<P>Aside from their ID, atom type, and xyz position, other properties of
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created atoms are set to default values, depending on which quantities
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are defined by the chosen <A HREF = "atom_style.html">atom style</A>. See the <A HREF = "atom_style.html">atom
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style</A> command for more details. See the
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<A HREF = "set.html">set</A> and <A HREF = "velocity.html">velocity</A> commands for info on how
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to change these values.
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</P>
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<UL><LI>charge = 0.0
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<LI>dipole moment = 0.0
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<LI>diameter = 1.0
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<LI>volume = 1.0
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<LI>density = 1.0
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<LI>velocity = 0.0
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<LI>angular velocity = 0.0
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<LI>angular momentum = 0.0
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<LI>quaternion = (1,0,0,0)
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<LI>bonds, angles, dihedrals, impropers = none
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</UL>
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<P>The <I>granular</I> style sets the diameter and density to 1.0 and
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calculates a mass for the particle, which is PI/6 * diameter^3 =
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0.5236. The <I>peri</I> style sets the volume and density to 1.0 and
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calculates a mass for the particle, which is also 1.0.
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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> must be previously defined to use this
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command.
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</P>
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<P><B>Related commands:</B>
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</P>
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2006-11-14 06:18:34 +08:00
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<P><A HREF = "lattice.html">lattice</A>, <A HREF = "region.html">region</A>, <A HREF = "create_box.html">create_box</A>,
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<A HREF = "read_data.html">read_data</A>, <A HREF = "read_restart.html">read_restart</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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