# pair_style mliap command Accelerator Variants: *mliap/kk* ## Syntax ``` LAMMPS pair_style mliap ... keyword values ... ``` - one or two keyword/value pairs must be appended - keyword = *model* or *descriptor* or *unified* *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 *unified* values = filename ghostneigh_flag filename = name of file containing serialized unified Python object ghostneigh_flag = 0/1 to turn off/on inclusion of ghost neighbors in neighbors list ## Examples ``` 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_style mliap unified mliap_unified_lj_Ar.pkl 0 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 straightforward to add new descriptor styles. By using the *unified* keyword, it is possible to define a Python model that combines functionalities of both *model* and *descriptor*. The SNAP descriptor style *sna* is the same as that used by [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 [compute mliap](compute_mliap) command. ::: versionadded 2Jun2022 ::: 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 [(Bartok)](Bartok2013) and [(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, or the one keyword *unified*. 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 [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 [(Yanxon)](Yanxon2020). ::: admonition Notes on mliappy models When the *model* keyword is *mliappy*, if the filename ends in \'.pt\', or \'.pth\', it will be loaded using pytorch; otherwise, it will be loaded as 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)]{.title-ref} from python before using the pair style. When using LAMMPS via the library mode, you will need to call [lammps.mliappy.activate_mliappy(lmp)]{.title-ref} 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 [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 [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 [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 [pair_coeff](pair_coeff) page for alternate ways to specify the path for these *model* and *descriptor* files. :::: note ::: title 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. :::: ::: versionadded 3Nov2022 ::: The *unified* keyword is followed by an argument specifying the filename containing the serialized unified Python object and the \"ghostneigh\" toggle (0/1) to disable/enable the construction of neighbors lists including neighbors of ghost atoms. If the filename ends in \'.pt\', or \'.pth\', it will be loaded using pytorch; otherwise, it will be loaded as a pickle file. If ghostneigh is enabled, it is recommended to set [comm_modify](comm_modify) cutoff manually, such as in the following example. ``` LAMMPS variable ninteractions equal 2 variable cutdist equal 7.5 variable skin equal 1.0 variable commcut equal (${ninteractions}*${cutdist})+${skin} neighbor ${skin} bin comm_modify cutoff ${commcut} ``` :::: note ::: title Note ::: To load a model from memory (i.e. an existing python object), call [lammps.mliap.load_unified(unified)]{.title-ref} from python, and then specify the filename as \"EXISTS\". When using LAMMPS via the library mode, you will need to call [lammps.mliappy.activate_mliappy(lmp)]{.title-ref} 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. :::: ------------------------------------------------------------------------ Styles with a *gpu*, *intel*, *kk*, *omp*, or *opt* 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 on the [Accelerator packages](Speed_packages) page. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues. These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the [Build package](Build_package) 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](Run_options) when you invoke LAMMPS, or you can use the [suffix](suffix) command in your input script. See the [Accelerator packages](Speed_packages) page for more instructions on how to use the accelerated styles effectively. ------------------------------------------------------------------------ ## 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 [pair_modify](pair_modify) shift, table, and tail options. This pair style does not write its information to [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 [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 [Build package](Build_package) page for more info. ## Related commands [pair_style snap](pair_snap), [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). :::