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### shape command

Syntax:

    shape I x y z

- I = atom type (see asterisk form below)
- x = x diameter (distance units)
- y = y diameter (distance units)
- z = z diameter (distance units)

Examples:

    shape 1 1.0 1.0 1.0
    shape * 3.0 1.0 1.0
    shape 2* 3.0 1.0 1.0

Description:

Set the shape for all atoms of one or more atom types. In LAMMPS, particles that have a finite size are said to have a "shape", as opposed to being a point mass. The shape can be spherical or aspherical, depending on whether the 3 shape values are the same or different. Shape values can also be set in the read_data data file using the "Shapes" keyword. See the units command for what distance units to use.

The I index can be specified in one of two ways. An explicit numeric value can be used, as in the 1st example above. Or a wild-card asterisk can be used to set the shape for multiple atom types. This takes the form "*" or "*n" or "n*" or "m*n". If N = the number of atom types, then an asterisk with no numeric values means all types from 1 to N. A leading asterisk means all types from 1 to n (inclusive). A trailing asterisk means all types from n to N (inclusive). A middle asterisk means all types from m to n (inclusive).

A line in a data file that follows the "Shapes" keyword specifies shape using the same format as the arguments of the shape command in an input script, except that no wild-card asterisk can be used. For example, under the "Shapes" section of a data file, the line that corresponds to the 1st example above would be listed as

    1 1.0 1.0 1.0

The shape values can be set to all 0.0, which means that atoms of that type are point particles and not finite-size particles. Some pair styles and fixes and computes that operate on finite-size particles allow for a mixture of finite-size and point particles. See the doc pages of individual commands for details.

Note that the shape command can only be used if the atom style requires per-type atom shape to be set. Currently, only the colloid, dipole, and ellipsoid styles do. The granular and peri styles also define finite-size spherical particles, but their size is set on a per-particle basis. These are are defined in the data file read by the read_data command, or set to default values by the create_atoms command, or set to new values by the set diameter command.

Dipoles use the atom shape to compute a moment of inertia for rotational energy. See the pair_style dipole command. Only the 1st component of the shape is used since the particles are assumed to be spherical.

Ellipsoids use the atom shape to compute a generalized inertia tensor. For example, a shape setting of 3.0 1.0 1.0 defines a particle 3x longer in x than in y or z and with a circular cross-section in yz. Ellipsoids which are in fact spherical can be defined by setting all 3 shape components the same.

If you define a hybrid atom style which includes one (or more) sub-styles which require per-type shape and one (or more) sub-styles which require per-atom diameter, then you must define both. However, in this case the per-type shape will be ignored; only the per-atom diameter will be used by LAMMPS. This means you cannot currently mix aspherical particles with per-atom diameter particles.

Restrictions:

This command must come after the simulation box is defined by a read_data, read_restart, or create_box command.

All shapes must be defined before a simulation is run (if the atom style requires shapes be set).

Related commands: none

Default: none