.. index:: pair_style mliap

pair_style mliap command
========================

Syntax
""""""

.. code-block:: LAMMPS

   pair_style mliap ... keyword values ...

* two keyword/value pairs must be appended
* keyword = *model* or *descriptor*

  .. parsed-literal::

       *model* values = style filename
         style = *linear* or *quadratic* or *nn* or *mliappy*
         filename = name of file containing model definitions
       *descriptor* values = style filename
         style = *sna* or *so3*
         filename = name of file containing descriptor definitions

Examples
""""""""

.. code-block:: LAMMPS

   pair_style mliap model linear InP.mliap.model descriptor sna InP.mliap.descriptor
   pair_style mliap model quadratic W.mliap.model descriptor sna W.mliap.descriptor
   pair_style mliap model nn Si.nn.mliap.model descriptor so3 Si.nn.mliap.descriptor
   pair_coeff * * In P

Description
"""""""""""

Pair style *mliap* provides a general interface to families of
machine-learning interatomic potentials. It allows separate definitions
of the interatomic potential functional form (*model*) and the geometric
quantities that characterize the atomic positions (*descriptor*). By
defining *model* and *descriptor* separately, it is possible to use many
different models with a given descriptor, or many different descriptors
with a given model. The pair style currently supports only *sna* and *so3*
descriptor styles, but it is is straightforward to add new descriptor
styles.

The SNAP descriptor style *sna* is the same as that used by
:doc:`pair_style snap <pair_snap>`, including the linear, quadratic, and
chem variants.  The available models are *linear*, *quadratic*, *nn*,
and *mliappy*.  The *mliappy* style can be used to couple python models,
e.g. PyTorch neural network energy models, and requires building LAMMPS
with the PYTHON package (see below).  In order to train a model, it is
useful to know the gradient or derivative of energy, force, and stress
w.r.t. model parameters. This information can be accessed using the
related :doc:`compute mliap <compute_mliap>` command.

The descriptor style *so3* is a descriptor that is derived from the
the smooth SO(3) power spectrum with the explicit inclusion of a radial
basis :ref:`(Bartok) <Bartok2013>` and :ref:`(Zagaceta) <Zagaceta2020>`.
The available models are *linear* and *nn*.

The pair_style *mliap* command must be followed by two keywords *model*
and *descriptor* in either order. A single *pair_coeff* command is also
required. The first 2 arguments must be \* \* so as to span all LAMMPS
atom types.  This is followed by a list of N arguments that specify the
mapping of MLIAP element names to LAMMPS atom types, where N is the
number of LAMMPS atom types.

The *model* keyword is followed by the model style. This is followed by
a single argument specifying the model filename containing the
parameters for a set of elements.  The model filename usually ends in
the *.mliap.model* extension.  It may contain parameters for many
elements. The only requirement is that it contain at least those element
names appearing in the *pair_coeff* command.

The top of the model file can contain any number of blank and comment
lines (start with #), but follows a strict format after that. The first
non-blank non-comment line must contain two integers:

* nelems  = Number of elements
* nparams = Number of parameters

When the *model* keyword is *linear* or *quadratic*, this is followed by
one block for each of the *nelem* elements.  Each block consists of
*nparams* parameters, one per line.  Note that this format is similar,
but not identical to that used for the :doc:`pair_style snap
<pair_snap>` coefficient file.  Specifically, the line containing the
element weight and radius is omitted, since these are handled by the
*descriptor*.

When the *model* keyword is *nn* (neural networks), the model file can
contain blank and comment lines (start with #) anywhere. The second
non-blank non-comment line must contain the string NET, followed by two
integers:

* ndescriptors = Number of descriptors
* nlayers      = Number of layers (including the hidden layers and the output layer)

and followed by a sequence of a string and an integer for each layer:

* Activation function (linear, sigmoid, tanh or relu)
* nnodes = Number of nodes

This is followed by one block for each of the *nelem* elements. Each
block consists of *scale0* minimum value, *scale1* (maximum - minimum)
value, in order to normalize the descriptors, followed by *nparams*
parameters, including *bias* and *weights* of the model, starting with
the first node of the first layer and so on, with a maximum of 30 values
per line.

The detail of *nn* module implementation can be found at :ref:`(Yanxon) <Yanxon2020>`.

.. admonition:: Notes on mliappy models

   When the *model* keyword is *mliappy*, the filename should end in '.pt',
   '.pth' for pytorch models, or be a pickle file. To load a model from
   memory (i.e. an existing python object), specify the filename as
   "LATER", and then call `lammps.mliap.load_model(model)` from python
   before using the pair style. When using lammps via the library mode, you
   will need to call `lammps.mliappy.activate_mliappy(lmp)` on the active
   lammps object before the pair style is defined. This call locates and
   loads the mliap-specific python module that is built into lammps.

The *descriptor* keyword is followed by a descriptor style, and additional arguments.
Currently two descriptor styles are available: *sna* and *so3*.

- *sna* indicates the bispectrum component descriptors used by the Spectral
  Neighbor Analysis Potential (SNAP) potentials of :doc:`pair_style snap
  <pair_snap>`.  A single additional argument specifies the descriptor
  filename containing the parameters and setting used by the SNAP
  descriptor.  The descriptor filename usually ends in the
  *.mliap.descriptor* extension.

- *so3* indicated the power spectrum component descriptors. A single additional
  argument specifies the descriptor filename containing the parameters and setting.

The SNAP descriptor file closely follows the format of the
:doc:`pair_style snap <pair_snap>` parameter file.  The file can contain
blank and comment lines (start with #) anywhere. Each non-blank
non-comment line must contain one keyword/value pair. The required
keywords are *rcutfac* and *twojmax*\ . There are many optional keywords
that are described on the :doc:`pair_style snap <pair_snap>` doc page.
In addition, the SNAP descriptor file must contain the *nelems*,
*elems*, *radelems*, and *welems* keywords.  The *nelems* keyword
specifies the number of elements provided in the other three keywords.
The *elems* keyword is followed by a list of *nelems* element names that
must include the element names appearing in the *pair_coeff* command,
but can contain other names too.  Similarly, the *radelems* and *welems*
keywords are followed by lists of *nelems* numbers giving the element
radius and element weight of each element. Obviously, the order in which
the elements are listed must be consistent for all three keywords.

The SO3 descriptor file is similar to the SNAP descriptor except that it
contains a few more arguments (e.g., *nmax* and *alpha*). The preparation
of SO3 descriptor and model files can be done with the
`Pyxtal_FF <https://github.com/qzhu2017/PyXtal_FF>`_ package.

See the :doc:`pair_coeff <pair_coeff>` page for alternate ways
to specify the path for these *model* and *descriptor* files.

.. note::

   To significantly reduce SO3 descriptor/force calculation time,
   some properties are pre-computed and reused during the calculation.
   These can consume a significant amount of RAM for simulations of
   larger systems since their size depends on the total number of
   neighbors per MPI process.

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 with
user-specifiable parameters as described above.  You never need to
specify a pair_coeff command with I != J arguments for this style.

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

This pair style does not write its information to :doc:`binary restart
files <restart>`, 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
:doc:`run_style respa <run_style>` command.  It does not support the
*inner*, *middle*, *outer* keywords.

----------

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

This pair style is part of the ML-IAP package.  It is only enabled if
LAMMPS was built with that package. In addition, building LAMMPS with
the ML-IAP package requires building LAMMPS with the ML-SNAP package.
The *mliappy* model requires building LAMMPS with the PYTHON package.
See the :doc:`Build package <Build_package>` page for more info.


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

:doc:`pair_style snap  <pair_snap>`, :doc:`compute mliap <compute_mliap>`

Default
"""""""

none

----------

.. _Bartok2013:

**(Bartok2013)** Bartok, Kondor, Csanyi, Phys Rev B, 87, 184115 (2013).

.. _Zagaceta2020:

**(Zagaceta2020)** Zagaceta, Yanxon, Zhu, J Appl Phys, 128, 045113 (2020).

.. _Yanxon2020:

**(Yanxon2020)** Yanxon, Zagaceta, Tang, Matteson, Zhu, Mach. Learn.: Sci. Technol. 2, 027001 (2020).


