"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c

:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Commands_all.html)

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pair_style atm command :h3

[Syntax:]

pair_style atm cutoff cutoff_triple :pre

cutoff = cutoff for each pair in 3-body interaction (distance units)
cutoff_triple = additional cutoff applied to product of 3 pairwise distances (distance units) :ul

[Examples:]

pair_style atm 4.5 2.5
pair_coeff * * * 0.072 :pre

pair_style hybrid/overlay lj/cut 6.5 atm 4.5 2.5
pair_coeff * * lj/cut 1.0 1.0
pair_coeff 1 1 atm 1 0.064
pair_coeff 1 1 atm 2 0.080
pair_coeff 1 2 atm 2 0.100
pair_coeff 2 2 atm 2 0.125 :pre

[Description:]

The {atm} style computes a 3-body "Axilrod-Teller-Muto"_#Axilrod
potential for the energy E of a system of atoms as

:c,image(Eqs/pair_atm.jpg)

where nu is the three-body interaction strength.  The distances
between pairs of atoms r12, r23, r31 and the angles gamma1, gamma2,
gamma3 are as shown in this diagram:

:c,image(JPG/pair_atm_dia.jpg)

Note that for the interaction between a triplet of atoms I,J,K, there
is no "central" atom.  The interaction is symmetric with respect to
permutation of the three atoms. Thus the nu value is
the same for all those permutations of the atom types of I,J,K
and needs to be specified only once, as discussed below.

The {atm} potential is typically used in combination with a two-body
potential using the "pair_style hybrid/overlay"_pair_hybrid.html
command as in the example above.

The potential for a triplet of atom is calculated only if all 3
distances r12, r23, r31 between the 3 atoms satisfy rIJ < cutoff.
In addition, the product of the 3 distances r12*r23*r31 <
cutoff_triple^3 is required, which excludes from calculation the
triplets with small contribution to the interaction.

The following coefficients must be defined for each pair of atoms
types via the "pair_coeff"_pair_coeff.html command as in the examples
above, or in the restart files read by the
"read_restart"_read_restart.html commands:

K = atom type of the third atom (1 to Ntypes)
nu = prefactor (energy/distance^9 units) :ul

K can be specified in one of two ways.  An explicit numeric value can
be used, as in the 2nd example above.  J <= K is required.  LAMMPS
sets the coefficients for the other 5 symmetric interactions to the
same values.  E.g. if I = 1, J = 2, K = 3, then these 6 values are set
to the specified nu: nu123, nu132, nu213, nu231, nu312, nu321.  This
enforces the symmetry discussed above.

A wildcard asterisk can be used for K to set the coefficients for
multiple triplets of 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).  Note that only type triplets with J <= K are
considered; if asterisks imply type triplets where K < J, they are
ignored.

Note that a pair_coeff command can override a previous setting for the
same I,J,K triplet.  For example, these commands set nu for all I,J.K
triplets, then overwrite nu for just the I,J,K = 2,3,4 triplet:

pair_coeff * * * 0.25
pair_coeff 2 3 4 0.1 :pre

Note that for a simulation with a single atom type, only a single
entry is required, e.g.

pair_coeff 1 1 1 0.25 :pre

For a simulation with two atom types, four pair_coeff commands will
specify all possible nu values:

pair_coeff 1 1 1 nu1
pair_coeff 1 1 2 nu2
pair_coeff 1 2 2 nu3
pair_coeff 2 2 2 nu4 :pre

For a simulation with three atom types, ten pair_coeff commands will
specify all possible nu values:

pair_coeff 1 1 1 nu1
pair_coeff 1 1 2 nu2
pair_coeff 1 1 3 nu3
pair_coeff 1 2 2 nu4
pair_coeff 1 2 3 nu5
pair_coeff 1 3 3 nu6
pair_coeff 2 2 2 nu7
pair_coeff 2 2 3 nu8
pair_coeff 2 3 3 nu9
pair_coeff 3 3 3 nu10 :pre

By default the nu value for all triplets is set to 0.0.  Thus it is
not required to provide pair_coeff commands that enumerate triplet
interactions for all K types.  If some I,J,K combination is not
specified, then there will be no 3-body ATM interactions for that
combination and all its permutations.  However, as with all pair
styles, it is required to specify a pair_coeff command for all I,J
combinations, else an error will result.

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[Mixing, shift, table, tail correction, restart, rRESPA info]:

This pair styles do not support the "pair_modify"_pair_modify.html
mix, shift, table, and tail options.

This pair style writes its information to "binary restart
files"_restart.html, so pair_style and pair_coeff commands do not need
to be specified in an input script that reads a restart file.
However, if the {atm} potential is used in combination with other
potentials using the "pair_style hybrid/overlay"_pair_hybrid.html
command then pair_coeff commands need to be re-specified
in the restart input script.

This pair style can only be used via the {pair} keyword of the
"run_style respa"_run_style.html command.  It does not support the
{inner}, {middle}, {outer} keywords.

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[Restrictions:]

This pair style is part of the MANYBODY package.  It is only enabled
if LAMMPS was built with that package.  See the "Build
package"_Build_package.html doc page for more info.

[Related commands:]

"pair_coeff"_pair_coeff.html

[Default:] none

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:link(Axilrod)
[(Axilrod)]
Axilrod and Teller, J Chem Phys, 11, 299 (1943);
Muto, Nippon Sugaku-Buturigakkwaishi 17, 629 (1943).
