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

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

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pair_style agni command :h3
pair_style agni/omp command :h3

[Syntax:]

pair_style agni :pre

[Examples:]
pair_style      agni
pair_coeff      * * Al.agni Al

[Description:]

Style {agni} style computes the manybody vectorial force components for
an atom as

:c,image(Eqs/pair_agni.jpg)

{u} labels the individual components, i.e. x, y or z, and {V} is the
corresponding atomic fingerprint. {d} is the Euclidean distance between
any two atomic fingerprints. A total of N_t reference atomic
environments are considered to construct the force field file. {alpha_t}
and {l} are the weight coefficients and length scale parameter of the
non-linear regression model.

The method implements the recently proposed machine learning access to
atomic forces as discussed extensively in the following publications -
"(Botu1)"_#Botu2015adaptive and "(Botu2)"_#Botu2015learning. The premise
of the method is to map the atomic environment numerically into a
fingerprint, and use machine learning methods to create a mapping to the
vectorial atomic forces.

Only a single pair_coeff command is used with the {agni} style which
specifies an AGNI potential file containing the parameters of the
force field for the needed elements. These are mapped to LAMMPS atom 
types by specifying N additional arguments after the filename in the 
pair_coeff command, where N is the number of LAMMPS atom types:

filename
N element names = mapping of AGNI elements to atom types :ul

See the "pair_coeff"_pair_coeff.html doc page for alternate ways
to specify the path for the force field file.

An AGNI force field is fully specified by the filename which contains the
parameters of the force field, i.e., the reference training environments
used to construct the machine learning force field. Example force field 
and input files are provided in the examples/USER/misc/agni directory. 

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Styles with {omp} suffix is 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 "Section 5"_Section_accelerate.html
of the manual.  The accelerated style takes the same arguments and
should produce the same results, except for round-off and precision
issues.

The accelerated style is part of the USER-OMP.  They are only enabled if
LAMMPS was built with those packages.  See the "Making
LAMMPS"_Section_start.html#start_3 section for more info.

You can specify the accelerated style explicitly in your input script
by including their suffix, or you can use the "-suffix command-line
switch"_Section_start.html#start_7 when you invoke LAMMPS, or you can
use the "suffix"_suffix.html command in your input script.

See "Section 5"_Section_accelerate.html of the manual for
more instructions on how to use the accelerated styles effectively.

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

This pair style does not support the "pair_modify"_pair_modify.html
shift, table, and tail options.

This pair style does not write its information to "binary restart
files"_restart.html, since it is stored in potential files.  Thus, you
need to re-specify the pair_style and pair_coeff commands in an input
script that reads a restart file.

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:]

Currently, only elemental systems are implemented. Also, the method only
provides access to the forces and not energies or stresses. However, one
can access the energy via thermodynamic integration of the forces as
discussed in "(Botu3)"_#Botu2016construct.  This pair style is part
of the USER-MISC package. It is only enabled if LAMMPS was built with
that package. See the "Making LAMMPS"_Section_start.html#start_3 section
for more info.

The AGNI force field files provided with LAMMPS (see the
potentials directory) are parameterized for metal "units"_units.html.
You can use the AGNI potential with any LAMMPS units, but you would need
to create your own AGNI potential file with coefficients listed in the
appropriate units if your simulation doesn't use "metal" units.

[Related commands:]

"pair_coeff"_pair_coeff.html

[Default:] none

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:link(Botu2015adaptive)
[(Botu1)] V. Botu and R. Ramprasad, Int. J. Quant. Chem., 115(16), 1074 (2015).

:link(Botu2015learning)
[(Botu2)] V. Botu and R. Ramprasad, Phys. Rev. B, 92(9), 094306 (2015).

:link(Botu2016construct)
[(Botu3)] V. Botu, R. Batra, J. Chapman and R. Ramprasad, https://arxiv.org/abs/1610.02098 (2016).
