LAMMPS website

Guidelines for posting questions to the LAMMPS forum

These ideas will help you diagnose some LAMMPS problems yourself. And they will make it easier for people who read your message to help figure out your problem.

Please note that the mail list gets a lot of traffic. While the last suggestion #10 invites general questions, you should not think of the list as a substitute for:

Here are the guidelines:

  1. Search the forum and mailing list archives here since your question may have come up previously. There is an "Advanced filters" option where you can further restrict matches and a dropdown list where you can choose the order of the search results.
  2. Put some descriptive text in the subject line. This helps people who search the archives.
  3. Select a suitable sub-category and apply tags (they will be created as needed) identifying the topic of your post. That helps grouping topics and makes browsing through the forum when looking for discussions of specific topics easier.
  4. State what version of LAMMPS you are using, which is a date. LAMMPS will usually print the version as the first line of output. In the source code it is found in the file "src/version.h". If you downloaded from the LAMMPS WWW site, then when you unpack the tarball it will create a directory (e.g. lammps-12Feb09) with a date which will likely be later than the major release date, since the download tarball is continually updated with bug fixes and new features decribed on this page. This date tells you the version you have is current up to and including the patch of the same date listed on this page. State this date.

    Reporting the LAMMPS version helps us significantly, because we often remember when a similar problem came up, and can tell if you are encountering a bug that has already been fixed.

    Note that if you are using a old version and your problem is something that many users would run into, like a bug with input or output, then we may tell you to upgrade to the current (fully patched) version to see if your problem goes away.

  5. If a LAMMPS command is not working as documented in the LAMMPS documentation pages on the WWW site, and you are using a non-current version of LAMMPS, then recognize that the doc pages on the WWW site always reflect the most-current, fully-patched version of LAMMPS. If you want to read doc pages consistent with the version you have, then browse the HTML files in the doc directory of your distribution.
  6. If you have a problem building LAMMPS or it doesn't recognize a command that is part of a package, then read the Build doc page carefully, including the Build_packages doc page. If you don't understand the instructions there because you are inexperienced with cmake or make, then you should find a local expert or sys admin who can help you get LAMMPS setup on your machine. This is because many build problems are hard to diagnose remotely. If you think there is a problem with LAMMPS itself (e.g. the compiler complains about a LAMMMPS source file), or you think others might have a Makefile for your target platform, then post a question.
  7. If you don't know what line in your input script is causing LAMMPS to generate an error, then use the -echo screen command-line option when you run LAMMPS.
  8. If you are having a problem with a specific input script command, carefully read the entire doc page associated with that command. There may be text somewhere on that page which answers your question.
  9. It's always a good idea to visualize your system, especially if it runs for a while before something goes wrong. Check that the initial configuration of atoms is what you intended, then that atoms move as you expect them to.

    If things go bad on a particular timestep, then print out lots of thermodynamics and dump lots of snapshots before that timestep. Things may be going bad earlier than you expect. This can usually be done easily by running near to the problem timestep, changing output settings, then running past the problem timestep. For big problems, you can write out a restart file before things go bad, then run from the restart file with more output enabled.

  10. Recognize that your post will be seen by many hundreds of people who follow, browse, or search the forum. Thus if your post is about a problem you are having with LAMMPS, it should be as simple and focused as possible. If you think there is a bug in LAMMPS, then describe exactly what occurred and why you expected the result to be diffferent.

    "Simple" means that posts like the following are not good:

    Please make an effort before you post, to isolate your problem and reproduce it with as small a physical problem (number of atoms, number of timesteps) and as short and simple an input script as possible. Does the problem occur on one processor or only in parallel?

    If you need to attach a data file for us to reproduce the problem, make it small. If it has to be big, then don't attach it to your email. Instead, tell us that you have it, and we'll ask you to email it to us directly if needed.

    The most important thing you can do for us, is to isolate your problem to one (or a few interacting) commands which cause the problem. Since you know your physical model and what you are trying to simulate, you can often figure this out more quickly than us, by trying variants of your input script that turn on/off various options.

  11. General questions are welcome such as:

    We will always answer the first kind of question. We may not answer the other kinds if we don't have any suggestions for you. Note that the last kind of question can be hard to answer and may not have anything to do with LAMMPS, but more to do with the potential, or what you are trying to measure, or how you are using molecular dynamics to perform your simulation.

    Finally, if in doubt, just post your question. You won't be the first to have posted a strange message or a question with an obvious answer.