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### fix deposit command

Syntax:

    fix ID group-ID deposit N type M seed keyword values ...

- ID, group-ID are documented in fix command
- deposit = style name of this fix command
- N = # of atoms to insert
- type = atom type to assign to inserted atoms
- M = insert a single particle every M steps
- seed = random # seed (positive integer)
- one or more keyword/value pairs may be appended to args
- keyword = region or id or global or local or near or attempt or rate or vx or vy or vz or units

        region value = region-ID
          region-ID = ID of region to use as insertion volume
        id value = max or next
          max = atom ID for new atom is max ID of all current atoms plus one
          next = atom ID for new atom increments by one for every deposition
        global values = lo hi
          lo,hi = put new particle a distance lo-hi above all other particles (distance units)
        local values = lo hi delta
          lo,hi = put new particle a distance lo-hi above any nearby particle beneath it (distance units)
          delta = lateral distance within which a neighbor is considered "nearby" (distance units)
        near value = R
          R = only insert particle if further than R from existing particles (distance units)
        attempt value = Q
          Q = attempt a single insertion up to Q times
        rate value = V
          V = z velocity (y in 2d) at which insertion volume moves (velocity units)
        vx values = vxlo vxhi
          vxlo,vxhi = range of x velocities for inserted particle (velocity units)
        vy values = vylo vyhi
          vylo,vyhi = range of y velocities for inserted particle (velocity units)
        vz values = vzlo vzhi
          vzlo,vzhi = range of z velocities for inserted particle (velocity units)
        target values = tx ty tz
          tx,ty,tz = location of sputtering target (distance units)
        units value = lattice or box
          lattice = the geometry is defined in lattice units
          box = the geometry is defined in simulation box units

Examples:

    fix 3 all deposit 1000 2 100 29494 region myblock local 1.0 1.0 1.0 units box
    fix 2 newatoms deposit 10000 1 500 12345 region disk near 2.0 vz -1.0 -0.8

    fix 4 sputter deposit 1000 2 500 12235 region sphere vz -1.0 -1.0 target 5.0 5.0 0.0 units lattice

Description:

Insert a single particle into the simulation domain every M timesteps until N particles have been inserted. This is useful for simulating the deposition of particles onto a surface.

Inserted particles have the specified atom type and are assigned to two groups: the default group "all" and the group specified in the fix deposit command (which can also be "all").

If you are computing temperature values which include inserted particles, you will want to use the compute_modify dynamic option, which insures the current number of atoms is used as a normalizing factor each time temperature is computed.

Care must be taken that inserted particles are not too near existing particles, using the options described below. When inserting particles above a surface in a non-periodic box (see the boundary command), the possibility of a particle escaping the surface and flying upward should be considered, since the particle may be lost or the box size may grow infinitely large. A fix wall/reflect command can be used to prevent this behavior. Note that if a shrink-wrap boundary is used, it is OK to insert the new particle outside the box, however the box will immediately be expanded to include the new particle. When simulating a sputtering experiment it is probably more realistic to ignore those atoms using the thermo_modify command with the lost ignore option and a fixed boundary.

This command must use the region keyword to define an insertion volume. The specified region must have been previously defined with a region command. It must be defined with side = in.

Each timestep a particle is to be inserted, its coordinates are chosen as follows. A random position within the insertion volume is generated. If neither the global or local keyword is used, that is the trial position. If the global keyword is used, the random x,y values are used, but the z position of the new particle is set above the highest current atom in the simulation by a distance randomly chosen between lo/hi. (For a 2d simulation, this is done for the y position.) If the local keyword is used, the z position is set a distance between lo/hi above the highest current atom in the simulation that is "nearby" the chosen x,y position. In this context, "nearby" means the lateral distance (in x,y) between the new and old particles is less than the delta parameter.

Once a trial x,y,z location has been computed, the insertion is only performed if no current particle in the simulation is within a distance R of the new particle. If this test fails, a new random position within the insertion volume is chosen and another trial is made. Up to Q attempts are made. If an atom is not successfully deposited, LAMMPS prints a warning message.

The rate option moves the insertion volume in the z direction (3d) or y direction (2d). This enables particles to be inserted from a successively higher height over time. Note that this parameter is ignored if the global or local keywords are used, since those options choose a z-coordinate for insertion independently.

The vx, vy, and vz components of velocity for the inserted particle are set using the values specified for the vx, vy, and vz keywords. Note that normally, new particles should be a assigned a negative vertical velocity so that they move towards the surface.

In case the target option is used, the velocity vector of the inserted particle will be changed in a way so that it would pass through the specified coordinate. This allows convenient simulation of a sputtering process.

The id keyword determines how an atom ID is assigned to newly deposited atoms. For the max setting, the IDs of all current atoms are checked and the new ID is the current maximum value plus one. This means that if atoms have left the system, the new ID may reflect this fact. For the next setting, the maximum ID of all atoms is stored at the time the fix is defined. Each time a deposited atom is added, this value is incremented by one and assigned to the new atom. Thus atom IDs for deposited atoms will be consecutive even if atoms leave the system over time.

The units keyword determines the meaning of the distance units used for the other deposition parameters. A box value selects standard distance units as defined by the units command, e.g. Angstroms for units = real or metal. A lattice value means the distance units are in lattice spacings. The lattice command must have been previously used to define the lattice spacing. Note that the units choice affects all the keyword values that have units of distance or velocity.

Restart, fix_modify, output, run start/stop, minimize info:

This fix writes the state of the deposition to binary restart files. This includes information about how many atoms have been depositied, the random number generator seed, the next timestep for deposition, etc. See the read_restart command for info on how to re-specify a fix in an input script that reads a restart file, so that the operation of the fix continues in an uninterrupted fashion.

None of the fix_modify options are relevant to this fix. No global or per-atom quantities are stored by this fix for access by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.

Restrictions:

The specified insertion region cannot be a "dynamic" region, as defined by the region command.

Related commands:

fix_pour, region

Default:

The option defaults are id = max, delta = 0.0, near = 0.0, attempt = 10, rate = 0.0, vx = 0.0 0.0, vy = 0.0 0.0, vz = 0.0 0.0, and units = lattice.