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### atom_style command

Syntax:

    atom_style style args

- style = angle or atomic or bond or charge or dipole or electron or ellipsoid or full or line or meso or molecular or peri or sphere or tri or hybrid

      args = none for any style except hybrid
      hybrid args = list of one or more sub-styles

Examples:

    atom_style atomic
    atom_style bond
    atom_style full
    atom_style hybrid charge bond

Description:

Define what style of atoms to use in a simulation. This determines what attributes are associated with the atoms. This command must be used before a simulation is setup via a read_data, read_restart, or create_box command.

Once a style is assigned, it cannot be changed, so use a style general enough to encompass all attributes. E.g. with style bond, angular terms cannot be used or added later to the model. It is OK to use a style more general than needed, though it may be slightly inefficient.

The choice of style affects what quantities are stored by each atom, what quantities are communicated between processors to enable forces to be computed, and what quantities are listed in the data file read by the read_data command.

These are the additional attributes of each style and the typical kinds of physical systems they are used to model. All styles store coordinates, velocities, atom IDs and types. See the read_data, create_atoms, and set commands for info on how to set these various quantities.

angle | bonds and angles | bead-spring polymers with stiffness
atomic | only the default values | coarse-grain liquids, solids, metals
bond | bonds | bead-spring polymers
charge | charge | atomic system with charges
dipole | charge and dipole moment | system with dipolar particles
electron | charge and spin and eradius | electronic force field
ellipsoid | shape, quaternion for particle orientation, angular momentum | extended aspherical particles
full | molecular + charge | bio-molecules
line | end points, angular velocity | rigid bodies
meso | rho, e, cv | SPH particles
molecular | bonds, angles, dihedrals, impropers | uncharged molecules
peri | mass, volume | mesocopic Peridynamic models
sphere | diameter, mass, angular velocity | granular models
tri | corner points, angular momentum | rigid bodies
wavepacket | charge, spin, eradius, etag, cs_re, cs_im | AWPMD

All of the styles assign mass to particles on a per-type basis, using the mass command, except for the finite-size particle styles discussed below. They assign mass on a per-atom basis.

All of the styles define point particles, except the sphere, ellipsoid, electron, peri, wavepacket, line, and tri styles, which define finite-size particles.

For the sphere style, the particles are spheres and each stores a per-particle diameter and mass. If the diameter > 0.0, the particle is a finite-size sphere. If the diameter = 0.0, it is a point particle.

For the ellipsoid style, the particles are ellipsoids and each stores a flag which indicates whether it is a finite-size ellipsoid or a point particle. If it is an ellipsoid, it also stores a shape vector with the 3 diamters of the ellipsoid and a quaternion 4-vector with its orientation.

For the electron style, the particles representing electrons are 3d Gaussians with a specified position and bandwidth or uncertainty in position, which is represented by the eradius = electron size.

For the peri style, the particles are spherical and each stores a per-particle mass and volume.

The meso style is for smoothed particle hydrodynamics (SPH) particles which store a density (rho), energy (e), and heat capacity (cv).

The wavepacket style is similar to electron, but the electrons may consist of several Gaussian wave packets, summed up with coefficients cs= (cs_re,cs_im). Each of the wave packets is treated as a separate particle in LAMMPS, wave packets belonging to the same electron must have identical etag values.

For the line style, the particles are idealized line segments and each stores a per-particle mass and length and orientation (i.e. the end points of the line segment).

For the tri style, the particles are planar triangles and each stores a per-particle mass and size and orientation (i.e. the corner points of the triangle).

Typically, simulations require only a single (non-hybrid) atom style. If some atoms in the simulation do not have all the properties defined by a particular style, use the simplest style that defines all the needed properties by any atom. For example, if some atoms in a simulation are charged, but others are not, use the charge style. If some atoms have bonds, but others do not, use the bond style.

The only scenario where the hybrid style is needed is if there is no single style which defines all needed properties of all atoms. For example, if you want dipolar particles which will be torqued and rotate, you would need to use "atom_style hybrid sphere dipole". When a hybrid style is used, atoms store and communicate the union of all quantities implied by the individual styles.

LAMMPS can be extended with new atom styles; see this section.

Restrictions:

This command cannot be used after the simulation box is defined by a read_data or create_box command.

The angle, bond, full, and molecular styles are part of the MOLECULAR package. The dipole style is part of the "dipole" package. The peri style is part of the PERI package for Peridynamics. The electron style is part of the USER-EFF package for electronic force fields. The meso style is part of the USER-SPH package for smoothed particle hydrodyanmics (SPH). See this PDF guide to using SPH in LAMMPS. The wavepacket style is part of the USER-AWPMD package for the antisymmetrized wave packet MD method. They are only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

Related commands:

read_data, pair_style

Default:

atom_style atomic