message command


message which protocol mode arg
  • which = client or server

  • protocol = md or mc

  • mode = file or zmq or mpi/one or mpi/two

    file arg = filename
      filename = file used for message exchanges
    zmq arg = socket-ID
      socket-ID for client = localhost:5555, see description below
      socket-ID for server = *:5555, see description below
    mpi/one arg = none
    mpi/two arg = filename
      filename = file used to establish communication between 2 MPI jobs


message client md file tmp.couple
message server md file tmp.couple

message client md zmq localhost:5555
message server md zmq *:5555

message client md mpi/one
message server md mpi/one

message client md mpi/two tmp.couple
message server md mpi/two tmp.couple


Establish a messaging protocol between LAMMPS and another code for the purpose of client/server coupling.

The Howto client/server doc page gives an overview of client/server coupling of LAMMPS with another code where one code is the “client” and sends request messages to a “server” code. The server responds to each request with a reply message. This enables the two codes to work in tandem to perform a simulation.

The which argument defines LAMMPS to be the client or the server.

The protocol argument defines the format and content of messages that will be exchanged between the two codes. The current options are:

  • md = run dynamics with another code

  • mc = perform Monte Carlo moves with another code

For protocol md, LAMMPS can be either a client or server. See the server md doc page for details on the protocol.

For protocol mc, LAMMPS can be the server. See the server mc doc page for details on the protocol.

The mode argument specifies how messages are exchanged between the client and server codes. Both codes must use the same mode and use consistent parameters.

For mode file, the 2 codes communicate via binary files. They must use the same filename, which is actually a file prefix. Several files with that prefix will be created and deleted as a simulation runs. The filename can include a path. Both codes must be able to access the path/file in a common filesystem.

For mode zmq, the 2 codes communicate via a socket on the server code’s machine. Support for socket messaging is provided by the open-source ZeroMQ library, which must be installed on your system. The client specifies an IP address (IPv4 format) or the DNS name of the machine the server code is running on, followed by a 4-digit port ID for the socket, separated by a colon. E.g.

localhost:5555        # client and server running on same machine      # server is  # server is

The server specifies “*:5555” where “*” represents all available interfaces on the server’s machine, and the port ID must match what the client specifies.


What are allowed port IDs?


Additional explanation is needed here about how to use the zmq mode on a parallel machine, e.g. a cluster with many nodes.

For mode mpi/one, the 2 codes communicate via MPI and are launched by the same mpirun command, e.g. with this syntax for OpenMPI:

mpirun -np 2 lmp_mpi -mpicolor 0 -in in.client -log log.client : -np 4 othercode args  # LAMMPS is client
mpirun -np 2 othercode args : -np 4 lmp_mpi -mpicolor 1 -in in.server  # LAMMPS is server

Note the use of the “-mpicolor color” command-line argument with LAMMPS. See the command-line args doc page for further explanation.

For mode mpi/two, the 2 codes communicate via MPI, but are launched be 2 separate mpirun commands. The specified filename argument is a file the 2 MPI processes will use to exchange info so that an MPI inter-communicator can be established to enable the 2 codes to send MPI messages to each other. Both codes must be able to access the path/file in a common filesystem.

Normally, the message command should be used at the top of a LAMMPS input script. It performs an initial handshake with the other code to setup messaging and to verify that both codes are using the same message protocol and mode. Assuming both codes are launched at (nearly) the same time, the other code should perform the same kind of initialization.

If LAMMPS is the client code, it will begin sending messages when a LAMMPS client command begins its operation. E.g. for the fix client/md command, it is when a run command is executed.

If LAMMPS is the server code, it will begin receiving messages when the server command is invoked.

A fix client command will terminate its messaging with the server when LAMMPS ends, or the fix is deleted via the unfix command. The server command will terminate its messaging with the client when the client signals it. Then the remainder of the LAMMPS input script will be processed.

If both codes do something similar, this means a new round of client/server messaging can be initiated after termination by re-using a 2nd message command in your LAMMPS input script, followed by a new fix client or server command.


This command is part of the MESSAGE package. It is only enabled if LAMMPS was built with that package. See the Build package doc page for more info.