forked from lijiext/lammps
146 lines
6.4 KiB
HTML
146 lines
6.4 KiB
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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>fix deposit command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID deposit N type M seed keyword values ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>deposit = style name of this fix command
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<LI>N = # of atoms to insert
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<LI>type = atom type to assign to inserted atoms
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<LI>M = insert a single particle every M steps
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<LI>seed = random # seed
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<LI>one or more keyword/value pairs may be appended to args
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<LI>keyword = <I>region</I> or <I>global</I> or <I>local</I> or <I>near</I> or <I>attempt</I> or <I>rate</I> or <I>vx</I> or <I>vy</I> or <I>vz</I> or <I>units</I>
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<PRE> <I>region</I> value = region-ID
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region-ID = ID of region to use as insertion volume
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<I>global</I> values = lo hi
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lo,hi = put new particle a distance lo-hi above all other particles (distance units)
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<I>local</I> values = lo hi delta
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lo,hi = put new particle a distance lo-hi above any nearby particle beneath it (distance units)
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delta = lateral distance within which a neighbor is considered "nearby" (distance units)
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<I>near</I> value = R
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R = only insert particle if further than R from existing particles (distance units)
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<I>attempt</I> value = Q
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Q = attempt a single insertion up to Q times
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<I>rate</I> value = V
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V = z velocity (y in 2d) at which insertion volume moves (velocity units)
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<I>vx</I> values = vxlo vxhi
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vxlo,vxhi = range of x velocities for inserted particle (velocity units)
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<I>vy</I> values = vylo vyhi
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vylo,vyhi = range of y velocities for inserted particle (velocity units)
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<I>vz</I> values = vzlo vzhi
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vzlo,vzhi = range of z velocities for inserted particle (velocity units)
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<I>units</I> value = <I>lattice</I> or <I>box</I>
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lattice = the geometry is defined in lattice units
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box = 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>fix 3 all deposit 1000 2 100 29494 region myblock local 1.0 1.0 1.0 units box
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fix 2 newatoms deposit 10000 1 500 12345 region disk near 2.0 vz -1.0 -0.8
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Insert a single particle into the simulation domain every M timesteps
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until N particles have been inserted. This is useful for simulating
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the deposition of particles onto a surface.
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</P>
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<P>Inserted particles have the specified atom type and are assigned to
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two groups: the default group "all" and the group specified in the fix
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deposit command (which can also be "all").
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</P>
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<P>If you are computing temperature values which include inserted
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particles, you will want to use the <A HREF = "temp_modify.html">temp_modify</A>
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dynamic option, which insures the current number of atoms is used as a
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normalizing factor each time temperature is computed.
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</P>
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<P>Care must be taken that inserted particles are not too near existing
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particles, using the options described below. When inserting
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particles above a surface in a non-perioidic box (see the
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<A HREF = "boundary.html">boundary</A> command), the possibility of a particle
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escaping the surface and flying upward should be considered, since the
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particle may be lost or the box size may grow infinitely large. A
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<A HREF = "fix_wall_reflect.html">fix wall/reflect</A> command can be used to
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prevent this behavior. Note that if a shrink-wrap boundary is used,
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it is OK to insert the new particle outside the box, however the box
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will immediately be expanded to include the new particle.
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</P>
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<P>This command must use the <I>region</I> keyword to define an insertion
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volume. The specified region must have been previously defined with a
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<A HREF = "region.html">region</A> command. It must be defined with side = <I>in</I>.
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</P>
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<P>Each timestep a particle is to be inserted, its coordinates are chosen
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as follows. A random position within the insertion volume is
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generated. If neither the <I>global</I> or <I>local</I> keyword is used, that
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is the trial position. If the <I>global</I> keyword is used, the random
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x,y values are used, but the z position of the new particle is set
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above the highest current atom in the simulation by a distance
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randomly chosen between lo/hi. (For a 2d simulation, this is done for
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the y position.) If the <I>local</I> keyword is used, the z position is
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set a distance between lo/hi above the highest current atom in the
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simulation that is "nearby" the chosen x,y position. In this context,
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"nearby" means the lateral distance (in x,y) between the new and old
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particles is less than the delta parameter.
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</P>
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<P>Once a trial x,y,z location has been computed, the insertion is only
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performed if no current particle in the simulation is within a
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distance R of the new particle. If this test fails, a new random
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position within the insertion volume is chosen and another trial is
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made. Up to Q attempts are made, after which LAMMPS prints a warning
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message.
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</P>
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<P>The <I>rate</I> option moves the insertion volume in the z direction (3d)
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or y direction (2d). This enables particles to be inserted from a
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successively higher height over time. Note that this parameter is
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ignored if the <I>global</I> or <I>local</I> keywords are used, since those
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options choose a z-coordinate for insertion independently.
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</P>
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<P>The vx, vy, and vz components of velocity for the inserted particle
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are set using the values specified for the <I>vx</I>, <I>vy</I>, and <I>vz</I>
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keywords. Note that normally, new particles should be a assigned a
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negative vertical velocity so that they move towards the surface.
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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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for the other deposition parameters. A <I>box</I> value selects standard
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distance units as defined by the <A HREF = "units.html">units</A> command,
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e.g. Angstroms for units = real or metal. A <I>lattice</I> value means the
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distance units are in lattice spacings. The <A HREF = "lattice.html">lattice</A>
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command must have been previously used to define the lattice spacing.
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Note that the units choice affects all the keyword values that have
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units of distance or velocity.
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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 = "fix_pour.html">fix_pour</A>, <A HREF = "region.html">region</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 delta = 0.0, near = 0.0, attempt = 10, rate =
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0.0, vx = 0.0 0.0, vy = 0.0 0.0, vz = 0.0 0.0, and units = lattice.
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</P>
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</HTML>
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