atom_style command¶

Syntax¶

atom_style style args

• style = angle or atomic or body or bond or charge or dipole or electron or ellipsoid or full or line or meso or molecular or peri or sphere or granular or bond/gran or superquadric or tri or hybrid or sph

args = none for any style except body and hybrid
body args = bstyle bstyle-args
  bstyle = style of body particles
  bstyle-args = additional arguments specific to the bstyle
                see the body doc page for details
hybrid args = list of one or more sub-styles, each with their args

Examples¶

atom_style atomic
atom_style bond
atom_style superquadric
atom_style full
atom_style body nparticle 2 10
atom_style hybrid charge bond
atom_style hybrid charge body nparticle 2 5

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.

All of the above styles define point particles, except the sphere, ellipsoid, electron, peri, wavepacket, line, tri, and body styles, which define finite-size particles. See Section_howto 14 for an overview of using finite-size particle models with LAMMPS.

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

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. This is typically used for granular models. Instead of sphere, keyword granular can be used.

For the bond/gran style, the number of granular bonds per atom is stored, and the information associated to it: the type of each bond, the ID of the bonded partner atom and the so-called bond history. The bond history is similar to the contact history for granular interaction, it stores the internal state of the bond. What exactly is stored in this internal state is defined by the granular bond style used. There are 2 parameters: The number of bond types, and the maximum number of bonds that each atom can have. For each bond type, the parameters have to be specified via the bond_coeff command (see example here ) Note that bond/gran is an experimental code which is may not be available in your release of LIGGGHTS. An example for the syntax is given below:

atom_style bond/gran n_bondtypes 1 bonds_per_atom 6

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 diameters 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).

For the body style, the particles are arbitrary bodies with internal attributes defined by the “style” of the bodies, which is specified by the bstyle argument. Body particles can represent complex entities, such as surface meshes of discrete points, collections of sub-particles, deformable objects, etc.

The body doc page describes the body styles LAMMPS currently supports, and provides more details as to the kind of body particles they represent. For all styles, each body particle stores moments of inertia and a quaternion 4-vector, so that its orientation and position can be time integrated due to forces and torques.

Note that there may be additional arguments required along with the bstyle specification, in the atom_style body command. These arguments are described in the body doc page.

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 rotate due to torque, you 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 as well as new body 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 MOLECULE package.

The line and tri styles are part of the ASPHERE package.

The body style is part of the BODY 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 hydrodynamics (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