LAMMPS driver

MaxwellLink ships a C++ fix mxl for LAMMPS so classical molecular dynamics jobs can exchange electric fields and dipole currents with the EM solver. The sources live in maxwelllink.mxl_drivers.lammps alongside a convenience installer.

Note

During the run the fix mxl connects to the SocketHub, receives the electric field, applies

\[\mathbf{F}_i = Q_i \widetilde{\mathbf{E}},\]

and returns

\[\mathrm{d}\boldsymbol{\mu}/\mathrm{d}t = \sum_i Q_i \mathbf{v}_i,\]

where \(Q_i\) and \(\mathbf{v}_i\) are the partial charge and velocity of atom \(i\).

Requirements

• POSIX-like environment (the fix uses BSD sockets).

• A LAMMPS build with atom IDs enabled and non-LJ units (such as units real or metal).

Usage

Socket preparation

On the MaxwellLink side create a socket hub, for example in Meep:

hub = mxl.SocketHub(host="0.0.0.0", port=31415, timeout=60.0)

LAMMPS build helper

Use the bundled script to build a LAMMPS executable that contains the fix:

mxl_install_lammps

The script clones LAMMPS, copies src/maxwelllink/mxl_drivers/lammps/fix_maxwelllink.cpp/.h into src/MISC, and compiles an lmp_mxl binary placed on PATH. Advanced users may copy the fix into an existing source tree and rebuild manually.

A modified LAMMPS code with this fix is also available at GitHub.

LAMMPS input

Add the fix to the group of atoms to be coupled:

fix 1 all mxl localhost 31415

Parameters

Name	Description
host	Hostname or IP address of the MaxwellLink process (third argument in the fix command). Required.
port	TCP port exposed by SocketHub. Must lie in (1024, 65536] for TCP mode. Required.
unix	Optional flag (append unix to the fix command) to connect via a Unix-domain socket at /tmp/socketmxl_<host>.
reset_dipole	Optional flag (append reset_dipole to the fix command) to subtract the molecular dipole moment by that at time zero.

Returned data

• time_au – Simulation time in atomic units.

• mux_au, muy_au, muz_au – Molecular dipole components in atomic units at half a time step after the force evaluation time.

• mux_m_au, muy_m_au, muz_m_au – Molecular dipole components in atomic units at the force evaluation time.

• energy_au – Sum of kinetic and potential energy in atomic units.

• temp_K – Instantaneous temperature in Kelvin.

• pe_au – Potential energy contribution in atomic units (currently zero with pairwise-only setups).

• ke_au – Kinetic energy contribution in atomic units.

Notes

• Ensure every atom has a charge (Q) defined; otherwise the external force is zero.

• The fix forces neighbor-list rebuilds every step, matching the requirement of exchanging fields at the FDTD cadence.

• See maxwelllink.mxl_drivers.lammps.install for scripting helpers that manage download and compilation.