.. index:: pair_style eam/apip
.. index:: pair_style eam/fs/apip

pair_style eam/apip command
=============================

Constant precision variant: *eam*

pair_style eam/fs/apip command
================================

Constant precision variant: *eam/fs*

Syntax
""""""

.. code-block:: LAMMPS

   pair_style eam/apip
   pair_style eam/fs/apip

Examples
""""""""

.. code-block:: LAMMPS

   pair_style hybrid/overlay eam/fs/apip pace/precise/apip lambda/input/csp/apip fcc cutoff 5.0 lambda/zone/apip 12.0
   pair_coeff * * eam/fs/apip Cu.eam.fs Cu
   pair_coeff * * pace/precise/apip Cu_precise.yace Cu
   pair_coeff * * lambda/input/csp/apip
   pair_coeff * * lambda/zone/apip


Description
"""""""""""
Style *eam* computes pairwise interactions for metals and metal alloys
using embedded-atom method (EAM) potentials :ref:`(Daw) <Daw2>`.  The total
energy :math:`E_i` of an atom :math:`i` is given by

.. math::

   E_i^\text{EAM} = F_\alpha \left(\sum_{j \neq i}\ \rho_\beta (r_{ij})\right) +
         \frac{1}{2} \sum_{j \neq i} \phi_{\alpha\beta} (r_{ij})

where :math:`F` is the embedding energy which is a function of the atomic
electron density :math:`\rho`, :math:`\phi` is a pair potential interaction,
and :math:`\alpha` and :math:`\beta` are the element types of atoms
:math:`i` and :math:`j`.  The multi-body nature of the EAM potential is a
result of the embedding energy term. Both summations in the formula are over
all neighbors :math:`j` of atom :math:`i` within the cutoff distance.
EAM is documented in detail in :doc:`pair_style eam <pair_eam>`.

The potential energy :math:`E_i` of an atom :math:`i` of an adaptive-precision
interatomic potential (APIP) according to :ref:`(Immel) <Immel2025_5>` is given by

.. math::

   E_i^\text{APIP} = \lambda_i E_i^\text{(fast)} + (1-\lambda_i) E_i^\text{(precise)}\,,

whereas the switching parameter :math:`\lambda_i` is computed
dynamically during a simulation by :doc:`fix lambda/apip <fix_lambda_apip>`
or set prior to a simulation via :doc:`set <set>`.

The pair style *eam/fs/apip* computes the potential energy
:math:`\lambda_i E_i^\text{EAM}` and the
corresponding force and should be combined
with a precise potential like
:doc:`pair_style pace/precise/apip <pair_pace_apip>` that computes the
potential energy :math:`(1-\lambda_i) E_i^\text{(precise)}` and the
corresponding force via :doc:`pair_style hybrid/overlay <pair_hybrid>`.

Mixing, shift, table, tail correction, restart, rRESPA info
"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""

For atom type pairs I,J and I != J, where types I and J correspond to
two different element types, mixing is performed by LAMMPS as
described above with the individual styles.  You never need to specify
a pair_coeff command with I != J arguments for the eam/apip styles.

This pair style does not support the :doc:`pair_modify <pair_modify>`
shift, table, and tail options.

The eam/apip pair styles do not write their information to :doc:`binary
restart files <restart>`, since it is stored in tabulated potential
files.  Thus, you need to re-specify the pair_style and pair_coeff
commands in an input script that reads a restart file.

The eam/apip pair styles can only be used via the *pair* keyword of the
:doc:`run_style respa <run_style>` command.  They do not support the
*inner*, *middle*, *outer* keywords.

----------

Restrictions
""""""""""""

This pair styles are part of the APIP package.  It is only enabled if
LAMMPS was built with that package.  See the :doc:`Build package
<Build_package>` page for more info.

Related commands
""""""""""""""""

:doc:`pair_style eam  <pair_eam>`,
:doc:`pair_style hybrid/overlay <pair_hybrid>`,
:doc:`fix lambda/apip <fix_lambda_apip>`,
:doc:`fix lambda_thermostat/apip <fix_lambda_thermostat_apip>`,
:doc:`pair_style lambda/zone/apip <pair_lambda_zone_apip>`,
:doc:`pair_style lambda/input/apip  <pair_lambda_input_apip>`,
:doc:`pair_style pace/apip <pair_pace_apip>`,
:doc:`fix atom_weight/apip <fix_atom_weight_apip>`

Default
"""""""

none

----------

.. _Immel2025_5:

**(Immel)** Immel, Drautz and Sutmann, J Chem Phys, 162, 114119 (2025)

.. _Daw2:

**(Daw)** Daw, Baskes, Phys Rev Lett, 50, 1285 (1983).
Daw, Baskes, Phys Rev B, 29, 6443 (1984).