atom_style command

Syntax

atom_style style args
  • style = angle or atomic or body or bond or charge or dipole or dpd or edpd or electron or ellipsoid or full or line or mdpd or molecular or peri or smd or sph or sphere or spin or tdpd or tri or template or hybrid

    args = none for any style except the following
      body args = bstyle bstyle-args
        bstyle = style of body particles
        bstyle-args = additional arguments specific to the bstyle
                      see the Howto body doc
                      page for details
      sphere arg = 0/1 (optional) for static/dynamic particle radii
      tdpd arg = Nspecies
        Nspecies = # of chemical species
      template arg = template-ID
        template-ID = ID of molecule template specified in a separate molecule command
      hybrid args = list of one or more sub-styles, each with their args
  • accelerated styles (with same args) = angle/kk or atomic/kk or bond/kk or charge/kk or full/kk or molecular/kk

Examples

atom_style atomic
atom_style bond
atom_style full
atom_style body nparticle 2 10
atom_style hybrid charge bond
atom_style hybrid charge body nparticle 2 5
atom_style spin
atom_style template myMols
atom_style hybrid template twomols charge
atom_style tdpd 2

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.

Note

Many of the atom styles discussed here are only enabled if LAMMPS was built with a specific package, as listed below in the Restrictions section.

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

body

mass, inertia moments, quaternion, angular momentum

arbitrary bodies

bond

bonds

bead-spring polymers

charge

charge

atomic system with charges

dipole

charge and dipole moment

system with dipolar particles

dpd

internal temperature and internal energies

DPD particles

edpd

temperature and heat capacity

eDPD particles

electron

charge and spin and eradius

electronic force field

ellipsoid

shape, quaternion, angular momentum

aspherical particles

full

molecular + charge

bio-molecules

line

end points, angular velocity

rigid bodies

mdpd

density

mDPD particles

mesont

mass, radius, length, buckling, connections, tube id

mesoscopic nanotubes

molecular

bonds, angles, dihedrals, impropers

uncharged molecules

peri

mass, volume

mesoscopic Peridynamic models

smd

volume, kernel diameter, contact radius, mass

solid and fluid SPH particles

sph

rho, esph, cv

SPH particles

sphere

diameter, mass, angular velocity

granular models

spin

magnetic moment

system with magnetic particles

tdpd

chemical concentration

tDPD particles

template

template index, template atom

small molecules with fixed topology

tri

corner points, angular momentum

rigid bodies

wavepacket

charge, spin, eradius, etag, cs_re, cs_im

AWPMD

Note

It is possible to add some attributes, such as a molecule ID, to atom styles that do not have them via the fix property/atom command. This command also allows new custom attributes consisting of extra integer or floating-point values to be added to atoms. See the fix property/atom doc page for examples of cases where this is useful and details on how to initialize, access, and output the custom values.

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 the Howto spherical doc page for an overview of using finite-size particle models with LAMMPS.

All of the point-particle styles assign mass to particles on a per-type basis, using the mass command, The finite-size particle styles 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. Note that by use of the disc keyword with the fix nve/sphere, fix nvt/sphere, fix nph/sphere, fix npt/sphere commands, spheres can be effectively treated as 2d discs for a 2d simulation if desired. See also the set density/disc command. The sphere style takes an optional 0 or 1 argument. A value of 0 means the radius of each sphere is constant for the duration of the simulation. A value of 1 means the radii may vary dynamically during the simulation, e.g. due to use of the fix adapt command.

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 dipole style, a point dipole is defined for each point particle. Note that if you wish the particles to be finite-size spheres as in a Stockmayer potential for a dipolar fluid, so that the particles can rotate due to dipole-dipole interactions, then you need to use atom_style hybrid sphere dipole, which will assign both a diameter and dipole moment to each particle.

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 dpd style is for dissipative particle dynamics (DPD) particles. Note that it is part of the USER-DPD package, and is not for use with the pair_style dpd or dpd/stat commands, which can simply use atom_style atomic. Atom_style dpd extends DPD particle properties with internal temperature (dpdTheta), internal conductive energy (uCond), internal mechanical energy (uMech), and internal chemical energy (uChem).

The edpd style is for energy-conserving dissipative particle dynamics (eDPD) particles which store a temperature (edpd_temp), and heat capacity(edpd_cv).

The mdpd style is for many-body dissipative particle dynamics (mDPD) particles which store a density (rho) for considering density-dependent many-body interactions.

The tdpd style is for transport dissipative particle dynamics (tDPD) particles which store a set of chemical concentration. An integer “cc_species” is required to specify the number of chemical species involved in a tDPD system.

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

The smd style is for a general formulation of Smooth Particle Hydrodynamics. Both fluids and solids can be modeled. Particles store the mass and volume of an integration point, a kernel diameter used for calculating the field variables (e.g. stress and deformation) and a contact radius for calculating repulsive forces which prevent individual physical bodies from penetrating each other.

For the spin style, a magnetic spin is associated to each atom. Those spins have a norm (their magnetic moment) and a direction.

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).

The template style allows molecular topology (bonds,angles,etc) to be defined via a molecule template using the molecule command. The template stores one or more molecules with a single copy of the topology info (bonds,angles,etc) of each. Individual atoms only store a template index and template atom to identify which molecule and which atom-within-the-molecule they represent. Using the template style instead of the bond, angle, molecular styles can save memory for systems comprised of a large number of small molecules, all of a single type (or small number of types). See the paper by Grime and Voth, in (Grime), for examples of how this can be advantageous for large-scale coarse-grained systems. The examples/template directory has a few demo inputs and examples showing the use of the template atom style versus molecular.

Note

When using the template style with a molecule template that contains multiple molecules, you should insure the atom types, bond types, angle_types, etc in all the molecules are consistent. E.g. if one molecule represents H2O and another CO2, then you probably do not want each molecule file to define 2 atom types and a single bond type, because they will conflict with each other when a mixture system of H2O and CO2 molecules is defined, e.g. by the read_data command. Rather the H2O molecule should define atom types 1 and 2, and bond type 1. And the CO2 molecule should define atom types 3 and 4 (or atom types 3 and 2 if a single oxygen type is desired), and bond type 2.

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 Howto 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 on the Howto 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, as mentioned above, 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.

When using the hybrid style, you cannot combine the template style with another molecular style that stores bond,angle,etc info on a per-atom basis.

LAMMPS can be extended with new atom styles as well as new body styles; see the Modify doc page.


Styles with a kk suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed in on the Speed packages doc page. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues.

Note that other acceleration packages in LAMMPS, specifically the GPU, USER-INTEL, USER-OMP, and OPT packages do not use accelerated atom styles.

The accelerated styles are part of the KOKKOS package. They are only enabled if LAMMPS was built with those packages. See the Build package doc page for more info.

You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the -suffix command-line switch when you invoke LAMMPS, or you can use the suffix command in your input script.

See the Speed packages doc page for more instructions on how to use the accelerated styles effectively.

Restrictions

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

Many of the styles listed above are only enabled if LAMMPS was built with a specific package, as listed below. See the Build package doc page for more info.

The angle, bond, full, molecular, and template 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 dpd style is part of the USER-DPD package for dissipative particle dynamics (DPD).

The edpd, mdpd, and tdpd styles are part of the USER-MESODPD package for energy-conserving dissipative particle dynamics (eDPD), many-body dissipative particle dynamics (mDPD), and transport dissipative particle dynamics (tDPD), respectively.

The sph style is part of the USER-SPH package for smoothed particle hydrodynamics (SPH). See this PDF guide to using SPH in LAMMPS.

The mesont style is part of the USER-MESONT package.

The spin style is part of the SPIN package.

The wavepacket style is part of the USER-AWPMD package for the antisymmetrized wave packet MD method.

Default

The default atom style is atomic. If atom_style sphere is used its default argument is 0.


(Grime) Grime and Voth, to appear in J Chem Theory & Computation (2014).