\(\renewcommand{\AA}{\text{Å}}\)

# pair_style mbx command

## Syntax

    pair_style mbx cutoff

- cutoff = real-space cutoff for MBX in Angstroms

## Examples

    pair_style      mbx 9.0
    pair_coeff      * * 1 h2o 1 2 2 json mbx.json
    compute         mbx all pair mbx

    # For a system involving ch4 (atom types C=1, H=2) and
    # water (atom types O=3, H=4)
    pair_style      mbx 9.0
    pair_coeff      * * 2 ch4 1 2 2 2 2 h2o 3 4 4 json mbx.json
    compute         mbx all pair mbx

    # For a system involving water (atom types O=12, H=13) in a hybrid simulation
    pair_style      hybrid/overlay mbx 9.0 lj/cut 9.0 coul/exclude 9.0
    pair_coeff      * * mbx 2 dp1 1*11 h2o 12 13 13 json mbx.json
    pair_coeff      1*11 1*11 coul/exclude
    compute         mbx all pair mbx

    # For a system involving water (atom types O=12, H=13) in a hybrid simulation
    # with special_bonds and coul/exclude to exclude 1-2, 1-3, and 1-4 electrostatics
    # for the charmm framework
    special_bonds   charmm
    pair_style      hybrid/overlay mbx 9.0 lj/cut 9.0 coul/exclude 9.0
    pair_coeff      * * mbx 2 dp1 1*11 h2o 12 13 13 json mbx.json
    pair_coeff      1*11 1*11 coul/exclude
    compute         mbx all pair mbx

See `examples/PACKAGES/mbx` for full examples of how to use MBX in LAMMPS.

## Description

Added in version 11Feb2026.

The MBX (Many-Body eXpansion) software is a C++ library that provides access to many-body energy (MB-nrg) potential energy functions, such as the MB-pol water model. Developed over the past decade, these potential energy functions integrate physics-based and machine-learned many-body terms trained on electronic structure data calculated at the “gold standard” coupled-cluster level of theory. (Gupta)

This pair_style instructs LAMMPS to call the MBX library in order to simulate MB-nrg models such as MB-pol. The MBX library source code is available at https://github.com/paesanilab/MBX. MBX is heavily OpenMP parallelized (OMP), and the OMP_NUM_THREADS environment variable should be properly set to the number of threads desired. A detailed discussion of the code structure can be found in the manuscript (Riera), while a detailed description of the performance scaling can be found in the manuscript (Gupta).

The cutoff argument specifies the real-space cutoff for MBX in Angstroms. This real-space cutoff is used for the dispersion interactions of the MB-nrg monomers, as well as for the electrostatics of the entire system. For periodic systems, a safe value for the real-space cutoff is 9.0 Angstroms, and all classical interactions beyond this cutoff will then be handled via particle-mesh Ewald (PME) within MBX. For non-periodic systems, the cutoff can be set to a large value, such as 100.0 Angstroms, to ensure that all interactions are captured in the real-space.

Warning

MBX must currently be used with processors mapping style xyz. If you do not, MBX will throw the error:

    [MBX] Inconsistent proc mapping: 'processors * * * map xyz' required for PME solver

For hybrid simulations involving MB-nrg and non-MB-nrg molecules in the same simulation, one can use pair_style hybrid/overlay to combine the MB-nrg molecules with other pair styles, such as lj/cut. This has been used to simulate MB-pol water within host frameworks such as metal-organic frameworks (MOFs) and carbon nanotubes (CNTs). If using MBX in a hybrid simulation involving special_bonds, (such as when using the CHARMM, Amber, OPLS, or ClayFF force fields etc.), please see the warning below for more details about using special_bonds with MBX dp1. See `examples/PACKAGES/mbx` for a complete hybrid example.

If you have questions not answered by this documentation, please reference the MBX website mbxsimulations.com or reach out to the MBX team at https://groups.google.com/g/mbx-users

## Pair coeff syntax

MBX is many-body method, and only a single pair_coeff command is needed to specify the mapping of LAMMPS atom IDs to MBX monomers. The syntax is as follows:

    pair_coeff * * num_mon_types mon_name atom_mapping <mon_name2> <atom_mapping2> ... json mbx.json print/settings

- num_mon_types = number of monomer types in the system

- mon_name = name of the monomer type (e.g. h2o, ch4, etc)

- atom mapping = list of LAMMPS atom IDs that correspond to the atoms in the monomer

- json arg = specifies the name of the MBX json configuration file, such as mbx.json

- print/settings = optionally print MBX settings to logfile

The num_mon_types argument specifies the number of different MB-nrg monomer types in the system.

For each monomer type, the mon_name argument specifies the name of the monomer, such as h2o for water or ch4 for methane. The atom mapping argument specifies then the mapping of LAMMPS atom IDs to the atoms in the monomer, such as 1 2 2 for water (O=1, H=2). For hybrid simulations, the dp1 (drude particle) monomer should be used to represent the non-MB-nrg molecules. dp1 is a special monomer in MBX in that its atom_mapping can be a range of LAMMPS atom IDs, such as 1*11 to represent atom types 1 through 11. For a complete list of available monomers in MBX, please see the MBX documentation.

Warning

When using MBX, all electrostatics are handled internally by MBX. This is important since MB-nrg models such as MB-pol are polarizable models that may also use geometrically dependent charges, such as the Partridge and Schwenke charges used in MB-pol water.

Therefore, one should never use a coulombic pair style in LAMMPS such as coul/cut or coul/long when also using MBX. This would result in double counting of electrostatic interactions.

When performing a hybrid simulation using dp1, note that many frameworks (Amber, CHARMM, OPLS, ClayFF etc.) require the usage of special_bonds to exclude some bonded coulomb interactions (1-2, 1-3, and/or 1-4). Since MBX is handling all electrostatics, this should therefore be accounted for using the coul/exclude command.

    # For a system involving water (atom types O=12, H=13) in a hybrid simulation
    # with special_bonds and coul/exclude to exclude 1-2, 1-3, and 1-4 electrostatics
    # for the charmm framework
    processors      * * * map xyz
    special_bonds   charmm
    pair_style      hybrid/overlay mbx 9.0 lj/cut 9.0 coul/exclude 9.0
    pair_coeff      * * mbx 2 dp1 1*11 h2o 12 13 13 json mbx.json
    pair_coeff      1*11 1*11 coul/exclude
    compute         mbx all pair mbx

The json argument specifies the name of the MBX JSON configuration file to use, such as mbx.json. If this file is not provided, the pair style will attempt to use a default configuration. See the mbx.json documentation for more details on how to create this file.

The print/settings argument optionally will print the MBX settings to the LAMMPS logfile at the start of the simulation. This is optionally used for debugging and ensuring that the settings are being correctly applied.

## Restrictions

This pair_style is part of the MBX package. A pair style is only enabled if LAMMPS was built with its corresponding package. See the Build package page for more info.

MBX requires the FFTW3 library to be installed. This is needed as part of the internal PME solver used for long-range electrostatics.

All electrostatic interactions are calculated internally in MBX. Therefore one should never calculate coulombic interactions in LAMMPS such as using coul/cut or coul/long when also using MBX. See the warning above for more details.

MBX currently only supports processors mapping style xyz.

MBX is primarily tested to work with units real and atom_style full. If you encounter issues with other unit or atom styles, please contact the MBX developers.

## Related commands

pair hybrid/overlay, pair coul/exclude

(Riera) M. Riera, C. Knight, E. Bull-Vulpe, X. Zhu, H. Agnew, D. Smith, A. Simmonett, F. Paesani, J. Chem. Phys. 159, 054802 (2023)

(Gupta) S. Gupta, E. Bull-Vulpe, H. Agnew, S. Iyer, X. Zhu, R. Zhou, C. Knight, F. Paesani, J. Chem. Theory Comput. 21, 1938 (2025)