Merge remote-tracking branch 'upstream/master'

# Contributing to LAMMPS via GitHub

Thank your for considering to contribute to the LAMMPS software project.

The following is a set of guidelines as well as explanations of policies and workflows for contributing to the LAMMPS molecular dynamics software project. These guidelines focus on submitting issues or pull requests on the LAMMPS GitHub project.

Thus please also have a look at:

* [The Section on submitting new features for inclusion in LAMMPS of the Manual](http://lammps.sandia.gov/doc/Section_modify.html#mod-15)

* [The LAMMPS GitHub Tutorial in the Manual](http://lammps.sandia.gov/doc/tutorial_github.html)

## Table of Contents

[I don't want to read this whole thing, I just have a question!](#i-dont-want-to-read-this-whole-thing-i-just-have-a-question)

[How Can I Contribute?](#how-can-i-contribute)

* [Discussing How To Use LAMMPS](#discussing-how-to-use-lammps)

* [Reporting Bugs](#reporting-bugs)

* [Suggesting Enhancements](#suggesting-enhancements)

* [Contributing Code](#contributing-code)

[GitHub Workflows](#github-workflows)

* [Issues](#issues)

* [Pull Requests](#pull-requests)

__

## I don't want to read this whole thing I just have a question!

> **Note:** Please do not file an issue to ask a general question about LAMMPS, its features, how to use specific commands, or how perform simulations or analysis in LAMMPS. Instead post your question to the ['lammps-users' mailing list](http://lammps.sandia.gov/mail.html). You do not need to be subscribed to post to the list (but a mailing list subscription avoids having your post delayed until it is approved by a mailing list moderator). Most posts to the mailing list receive a response within less than 24 hours. Before posting to the mailing list, please read the [mailing list guidelines](http://lammps.sandia.gov/guidelines.html). Following those guidelines will help greatly to get a helpful response. Always mention which LAMMPS version you are using.

## How Can I Contribute?

There are several ways how you can actively contribute to the LAMMPS project: you can discuss compiling and using LAMMPS, and solving LAMMPS related problems with other LAMMPS users on the lammps-users mailing list, you can report bugs or suggest enhancements by creating issues on GitHub (or posting them to the lammps-users mailing list), and you can contribute by submitting pull requests on GitHub or e-mail your code

to one of the [LAMMPS core developers](http://lammps.sandia.gov/authors.html). As you may see from the aforementioned developer page, the LAMMPS software package includes the efforts of a very large number of contributors beyond the principal authors and maintainers.

### Discussing How To Use LAMMPS

The LAMMPS mailing list is hosted at SourceForge. The mailing list began in 2005, and now includes tens of thousands of messages in thousands of threads. LAMMPS developers try to respond to posted questions in a timely manner, but there are no guarantees. Please consider that people live in different timezone and may not have time to answer e-mails outside of their work hours.

You can post to list by sending your email to lammps-users at lists.sourceforge.net (no subscription required), but before posting, please read the [mailing list guidelines](http://lammps.sandia.gov/guidelines.html) to maximize your chances to receive a helpful response.

Anyone can browse/search previous questions/answers in the archives. You do not have to subscribe to the list to post questions, receive answers (to your questions), or browse/search the archives. You **do** need to subscribe to the list if you want emails for **all** the posts (as individual messages or in digest form), or to answer questions yourself. Feel free to sign up and help us out! Answering questions from fellow LAMMPS users is a great way to pay back the community for providing you a useful tool for free, and to pass on the advice you have received yourself to others. It improves your karma and helps you understand your own research better.

If you post a message and you are a subscriber, your message will appear immediately. If you are not a subscriber, your message will be moderated, which typically takes one business day. Either way, when someone replies the reply will usually be sent to both, your personal email address and the mailing list. When replying to people, that responded to your post to the list, please always included the mailing list in your replies (i.e. use "Reply All" and **not** "Reply"). Responses will appear on the list in a few minutes, but it can take a few hours for postings and replies to show up in the SourceForge archive. Sending replies also to the mailing list is important, so that responses are archived and people with a similar issue can search for possible solutions in the mailing list archive.

### Reporting Bugs

While developers writing code for LAMMPS are careful to test their code, LAMMPS is such a large and complex software, that it is impossible to test for all combinations of features under all normal and not so normal circumstances. Thus bugs do happen, and if you suspect, that you have encountered one, please try to document it and report it as an [Issue](https://github.com/lammps/lammps/issues) on the LAMMPS GitHub project web page. However, before reporting a bug, you need to check whether this is something that may have already been corrected. The [Latest Features and Bug Fixes in LAMMPS](http://lammps.sandia.gov/bug.html) web page lists all significant changes to LAMMPS over the years. It also tells you what the current latest development version of LAMMPS is, and you should test whether your issue still applies to that version.

When you click on the green "New Issue" button, you will be provided with a text field, where you can enter your message. That text field with contain a template with several headlines and some descriptions. Keep the headlines that are relevant to your reported potential bug and replace the descriptions with the information as suggested by the descriptions.

You can also attach small text files (please add the file name extension `.txt` or it will be rejected), images, or small compressed text files (using gzip, do not use RAR or 7-ZIP or similar tools that are uncommon outside of Windows machines). In many cases, bugs are best illustrated by providing a small input deck (do **not** attach your entire production input, but remove everything that is not required to reproduce the issue, and scale down your system size, that the resulting calculation runs fast and can be run on small desktop quickly).

To be able to submit an issue on GitHub, you have to register for an account (for GitHub in general). If you do not want to do that, or have other reservations against submitting an issue there, you can - as an alternative and in decreasing preference - either send an e-mail to the lammps-users mailing list, the original authors of the feature that you suspect to be affected, or one or more of the core LAMMPS developers.

### Suggesting Enhancements

The LAMMPS developers welcome suggestions for enhancements or new features. These should be submitted using the [GitHub Issue Tracker](https://github.com/lammps/lammps/issues) of the LAMMPS project. This is particularly recommended, when you plan to implement the feature or enhancement yourself, as this allows to coordinate in case there are other similar or conflicting ongoing developments.

The LAMMPS developers will review your submission and consider implementing it. Whether this will actually happen depends on many factors: how difficult it would be, how much effort it would take, how many users would benefit from it, how well the individual developer would understand the underlying physics of the feature, and whether this is a feature that would fit into a software like LAMMPS, or would be better implemented as a separate tool. Because of these factors, it matters how well the suggested enhancement is formulated and the overall benefit is argued convincingly.

To be able to submit an issue on GitHub, you have to register for an account (for GitHub in general). If you do not want to do that, or have other reservations against submitting an issue there, you can - as an alternative - send an e-mail to the lammps-users mailing list.

### Contributing Code

We encourage users to submit new features or modifications for LAMMPS to the core developers so they can be added to the LAMMPS distribution. The preferred way to manage and coordinate this is by submitting a pull request at the LAMMPS project on GitHub. For any larger modifications or programming project, you are encouraged to contact the LAMMPS developers ahead of time, in order to discuss implementation strategies and coding guidelines, that will make it easier to integrate your contribution and result in less work for everybody involved. You are also encouraged to search through the list of open issues on GitHub and submit a new issue for a planned feature, so you would not duplicate the work of others (and possibly get scooped by them) or have your work duplicated by others.

How quickly your contribution will be integrated depends largely on how much effort it will cause to integrate and test it, how much it requires changes to the core code base, and of how much interest it is to the larger LAMMPS community. Please see below for a checklist of typical requirements. Once you have prepared everything, see [this tutorial](http://lammps.sandia.gov/doc/tutorial_github.html)

 for instructions on how to submit your changes or new files through a GitHub pull request

Here is a checklist of steps you need to follow to submit a single file or user package for our consideration. Following these steps will save both you and us time. See existing files in packages in the source directory for examples. If you are uncertain, please ask on the lammps-users mailing list.

* All source files you provide must compile with the most current version of LAMMPS with multiple configurations. In particular you need to test compiling LAMMPS from scratch with `-DLAMMPS_BIGBIG` set in addition to the default `-DLAMMPS_SMALLBIG` setting. Your code will need to work correctly in serial and in parallel using MPI.

* For consistency with the rest of LAMMPS and especially, if you want your contribution(s) to be added to main LAMMPS code or one of its standard packages, it needs to be written in a style compatible with other LAMMPS source files. This means: 2-character indentation per level, no tabs, no lines over 80 characters. I/O is done via the C-style stdio library, class header files should not import any system headers outside <stdio.h>, STL containers should be avoided in headers, and forward declarations used where possible or needed. All added code should be placed into the LAMMPS_NS namespace or a sub-namespace; global or static variables should be avoided, as they conflict with the modular nature of LAMMPS and the C++ class structure. Header files must not import namespaces with using. This all is so the developers can more easily understand, integrate, and maintain your contribution and reduce conflicts with other parts of LAMMPS. This basically means that the code accesses data structures, performs its operations, and is formatted similar to other LAMMPS source files, including the use of the error class for error and warning messages.

* If you want your contribution to be added as a user-contributed feature, and it is a single file (actually a `<name>.cpp` and `<name>.h` file) it can be rapidly added to the USER-MISC directory. Include the one-line entry to add to the USER-MISC/README file in that directory, along with the 2 source files. You can do this multiple times if you wish to contribute several individual features.

* If you want your contribution to be added as a user-contribution and it is several related features, it is probably best to make it a user package directory with a name like USER-FOO. In addition to your new files, the directory should contain a README text file. The README should contain your name and contact information and a brief description of what your new package does. If your files depend on other LAMMPS style files also being installed (e.g. because your file is a derived class from the other LAMMPS class), then an Install.sh file is also needed to check for those dependencies. See other README and Install.sh files in other USER directories as examples. Send us a tarball of this USER-FOO directory.

* Your new source files need to have the LAMMPS copyright, GPL notice, and your name and email address at the top, like other user-contributed LAMMPS source files. They need to create a class that is inside the LAMMPS namespace. If the file is for one of the USER packages, including USER-MISC, then we are not as picky about the coding style (see above). I.e. the files do not need to be in the same stylistic format and syntax as other LAMMPS files, though that would be nice for developers as well as users who try to read your code.

* You **must** also create or extend a documentation file for each new command or style you are adding to LAMMPS. For simplicity and convenience, the documentation of groups of closely related commands or styles may be combined into a single file. This will be one file for a single-file feature. For a package, it might be several files. These are simple text files with a specific markup language, that are then auto-converted to HTML and PDF. The tools for this conversion are included in the source distribution, and the translation can be as simple as doing "make html pdf" in the doc folder. Thus the documentation source files must be in the same format and style as other `<name>.txt` files in the lammps/doc/src directory for similar commands and styles; use one or more of them as a starting point. A description of the markup can also be found in `lammps/doc/utils/txt2html/README.html` As appropriate, the text files can include links to equations (see doc/Eqs/*.tex for examples, we auto-create the associated JPG files), or figures (see doc/JPG for examples), or even additional PDF files with further details (see doc/PDF for examples). The doc page should also include literature citations as appropriate; see the bottom of doc/fix_nh.txt for examples and the earlier part of the same file for how to format the cite itself. The "Restrictions" section of the doc page should indicate that your command is only available if LAMMPS is built with the appropriate USER-MISC or USER-FOO package. See other user package doc files for examples of how to do this. The prerequisite for building the HTML format files are Python 3.x and virtualenv, the requirement for generating the PDF format manual is the htmldoc software. Please run at least "make html" and carefully inspect and proofread the resulting HTML format doc page before submitting your code.

* For a new package (or even a single command) you should include one or more example scripts demonstrating its use. These should run in no more than a couple minutes, even on a single processor, and not require large data files as input. See directories under examples/USER for examples of input scripts other users provided for their packages. These example inputs are also required for validating memory accesses and testing for memory leaks with valgrind

* If there is a paper of yours describing your feature (either the algorithm/science behind the feature itself, or its initial usage, or its implementation in LAMMPS), you can add the citation to the *.cpp source file. See src/USER-EFF/atom_vec_electron.cpp for an example. A LaTeX citation is stored in a variable at the top of the file and a single line of code that references the variable is added to the constructor of the class. Whenever a user invokes your feature from their input script, this will cause LAMMPS to output the citation to a log.cite file and prompt the user to examine the file. Note that you should only use this for a paper you or your group authored. E.g. adding a cite in the code for a paper by Nose and Hoover if you write a fix that implements their integrator is not the intended usage. That kind of citation should just be in the doc page you provide.

Finally, as a general rule-of-thumb, the more clear and self-explanatory you make your documentation and README files, and the easier you make it for people to get started, e.g. by providing example scripts, the more likely it is that users will try out your new feature.

If the new features/files are broadly useful we may add them as core files to LAMMPS or as part of a standard package. Else we will add them as a user-contributed file or package. Examples of user packages are in src sub-directories that start with USER. The USER-MISC package is simply a collection of (mostly) unrelated single files, which is the simplest way to have your contribution quickly added to the LAMMPS distribution. You can see a list of the both standard and user packages by typing "make package" in the LAMMPS src directory.

Note that by providing us files to release, you are agreeing to make them open-source, i.e. we can release them under the terms of the GPL, used as a license for the rest of LAMMPS. See Section 1.4 for details.

With user packages and files, all we are really providing (aside from the fame and fortune that accompanies having your name in the source code and on the Authors page of the LAMMPS WWW site), is a means for you to distribute your work to the LAMMPS user community, and a mechanism for others to easily try out your new feature. This may help you find bugs or make contact with new collaborators. Note that you are also implicitly agreeing to support your code which means answer questions, fix bugs, and maintain it if LAMMPS changes in some way that breaks it (an unusual event).

To be able to submit an issue on GitHub, you have to register for an account (for GitHub in general). If you do not want to do that, or have other reservations or difficulties to submit a pull request, you can - as an alternative - contact one or more of the core LAMMPS developers and ask if one of them would be interested in manually merging your code into LAMMPS and send them your source code. Since the effort to merge a pull request is a small fraction of the effort of integrating source code manually (which would usually be done by converting the contribution into a pull request), your chances to have your new code included quickly are the best with a pull request.

If you prefer to submit patches or full files, you should first make certain, that your code works correctly with the latest patch-level version of LAMMPS and contains all bug fixes from it. Then create a gzipped tar file of all changed or added files or a corresponding patch file using 'diff -u' or 'diff -c' and compress it with gzip. Please only use gzip compression, as this works well on all platforms.

## GitHub Workflows

This section briefly summarizes the steps that will happen **after** you have submitted either an issue or a pull request on the LAMMPS GitHub project page. 

### Issues

After submitting an issue, one or more of the LAMMPS developers will review it and categorize it by assigning labels. Confirmed bug reports will be labeled `bug`; if the bug report also contains a suggestion for how to fix it, it will be labeled `bugfix`; if the issue is a feature request, it will be labeled `enhancement`. Other labels may be attached as well, depending on which parts of the LAMMPS code are affected. If the assessment is, that the issue does not warrant any changes, the `wontfix` label will be applied and if the submission is incorrect or something that should not be submitted as an issue, the `invalid` label will be applied. In both of the last two cases, the issue will then be closed without further action.

For feature requests, what happens next is that developers may comment on the viability or relevance of the request, discuss and make suggestions for how to implement it. If a LAMMPS developer or user is planning to implement the feature, the issue will be assigned to that developer. For developers, that are not yet listed as LAMMPS project collaborators, they will receive an invitation to be added to the LAMMPS project as a collaborator so they can get assigned. If the requested feature or enhancement is implemented, it will usually be submitted as a pull request, which will contain a reference to the issue number. And once the pull request is reviewed and accepted for inclusion into LAMMPS, the issue will be closed. For details on how pull requests are processed, please see below.

For bug reports, the next step is that one of the core LAMMPS developers will self-assign to the issue and try to confirm the bug. If confirmed, the `bug` label and potentially other labels are added to classify the issue and its impact to LAMMPS. Before confirming, further questions may be asked or requests for providing additional input files or details about the steps required to reproduce the issue. Any bugfix is likely to be submitted as a pull request (more about that below) and since most bugs require only local changes, the bugfix may be included in a pull request specifically set up to collect such local bugfixes or small enhancements. Once the bugfix is included in the master branch, the issue will be closed.

### Pull Requests

For submitting pull requests, there is a [detailed tutorial](http://lammps.sandia.gov/doc/tutorial_github.html) in the LAMMPS manual. Thus only a brief breakdown of the steps is presented here.

Immediately after the submission, the LAMMPS continuing integration server at ci.lammps.org will download your submitted branch and perform a simple compilation test, i.e. will test whether your submitted code can be compiled under various conditions. It will also do a check on whether your included documentation translates cleanly. Whether these tests are successful or fail will be recorded. If a test fails, please inspect the corresponding output on the CI server and take the necessary steps, if needed, so that the code can compile cleanly again. The test will be re-run each the pull request is updated with a push to the remote branch on GitHub.

Next a LAMMPS core developer will self-assign and do an overall technical assessment of the submission. If you are not yet registered as a LAMMPS collaborator, you will receive an invitation for that.

You may also receive comments and suggestions on the overall submission or specific details. If permitted, additional changes may be pushed into your pull request branch or a pull request may be filed in your LAMMPS fork on GitHub to include those changes.

The LAMMPS developer may then decide to assign the pull request to another developer (e.g. when that developer is more knowledgeable about the submitted feature or enhancement or has written the modified code). It may also happen, that additional developers are requested to provide a review and approve the changes. For submissions, that may change the general behavior of LAMMPS, or where a possibility of unwanted side effects exists, additional tests may be requested by the assigned developer.

If the assigned developer is satisfied and considers the submission ready for inclusion into LAMMPS, the pull request will be assigned to the LAMMPS lead developer, Steve Plimpton (@sjplimp), who will then have the final decision on whether the submission will be included, additional changes are required or it will be ultimately rejected. After the pull request is merged, you may delete the pull request branch in your personal LAMMPS fork.

Since the learning curve for git is quite steep for efficiently managing remote repositories, local and remote branches, pull requests and more, do not hesitate to ask questions, if you are not sure about how to do certain steps that are asked of you. Even if the changes asked of you do not make sense to you, they may be important for the LAMMPS developers. Please also note, that these all are guidelines and not set in stone.

## Purpose

_Briefly describe the new feature(s), enhancement(s), or bugfix(es) included in this pull request. If this addresses an open GitHub Issue, mention the issue number, e.g. with `fixes #221` or `closes #135`, so that issue will be automatically closed when the pull request is merged_

## Author(s)

_Please state name and affiliation of the author or authors that should be credited with the changes in this pull request_

## Backward Compatibility

_Please state whether any changes in the pull request break backward compatibility for inputs, and - if yes - explain what has been changed and why_

## Implementation Notes

_Provide any relevant details about how the changes are implemented, how correctness was verified, how other features - if any - in LAMMPS are affected_

## Post Submission Checklist

_Please check the fields below as they are completed_

- [ ] The feature or features in this pull request is complete

- [ ] Suitable new documentation files and/or updates to the existing docs are included

- [ ] One or more example input decks are included

- [ ] The source code follows the LAMMPS formatting guidelines

## Further Information, Files, and Links

_Put any additional information here, attach relevant text or image files, and URLs to external sites (e.g. DOIs or webpages)_