message command


message which protocol mode arg
  • which = client or server or quit

  • 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

message quit


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.

As explained below the quit option should be used when LAMMPS is finished as a client. It sends a message to the server to tell it to shut down.

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 or 5 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.


On Linux or Unix machines port IDs below 1024 are reserved to the superuser and thus not available. Other ports may already be in use and cannot be opened by a second process. On a Linux machine the commands “netstat -t4an” or “ss -t4an” will list all locally used port IDs for IPv4 addresses.


On many machines (and sometimes on local networks) also ports IDs may be blocked by default through firewalls. In that case either access to the required port (or a desired range of ports) has to be selectively enabled to the firewall disabled (the latter is usually not a good idea unless you are on a (small) local network that is already protected from outside access.


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 client or message server 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.

If LAMMPS is being used as a client, the message quit command will terminate its messaging with the server. If you do not use this command and just allow LAMMPS to exit, then the server will continue to wait for further messages. This may not be a problem, but if both the client and server programs were launched in the same batch script, then if the server runs indefinitely, it may consume the full allocation of computer time, even if the calculation finishes sooner.

Note that if LAMMPS is the client or server, it will continue processing the rest of its input script after client/server communication terminates.

If both codes cooperate in this manner, a new round of client/server messaging can be initiated after termination by re-using a second message command in your LAMMPS input script, followed by a new fix client or server command, followed by another message quit command (if LAMMPS is the client). As an example, this can be performed in a loop to use a quantum code as a server to compute quantum forces for multiple LAMMPS data files or periodic snapshots while running dynamics.


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.