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# 3.1. Overview

The best way to add a new feature to LAMMPS is to find a similar feature and look at the corresponding source and header files to figure out what it does. You will need some knowledge of C++ to understand the high-level structure of LAMMPS and its class organization. Functions (class methods) that do actual computations are mostly written in C-style code and operate on simple C-style data structures (vectors and arrays). A high-level overview of the programming style choices in LAMMPS is given elsewhere.

Most of the new features described on the Modify doc page require you to write a new C++ derived class (excluding exceptions described below, this can often be done by making small edits to existing files). Creating a new class requires 2 files, a source code file (*.cpp) and a header file (*.h). The derived class must provide certain methods to work as a new option. Depending on how different your new feature is compared to existing features, you can either derive from the base class itself, or from a derived class that already exists. Enabling LAMMPS to invoke the new class is as simple as putting the two source files in the src directory and re-building LAMMPS.

The advantage of C++ and its object-orientation is that all the code and variables needed to define the new feature are in the 2 files you write. Thus, it should not make the rest of LAMMPS more complex or cause bugs through unwanted side effects.

Here is a concrete example. Suppose you write 2 files `pair_foo.cpp` and `pair_foo.h` that define a new class `PairFoo` which computes pairwise potentials described in the classic 1997 paper by Foo, et al. If you wish to invoke those potentials in a LAMMPS input script with a command like:

    pair_style foo 0.1 3.5

then your `pair_foo.h` file should be structured as follows:

    #ifdef PAIR_CLASS
    // clang-format off
    PairStyle(foo,PairFoo);
    #else
    // clang-format on
    ...
    (class definition for PairFoo)
    ...
    #endif

where “foo” is the style keyword in the pair_style command, and `PairFoo` is the class name defined in your `pair_foo.cpp` and `pair_foo.h` files.

When you re-build LAMMPS, your new pairwise potential becomes part of the executable and can be invoked with a pair_style command like the example above. Arguments like 0.1 and 3.5 can be defined and processed by your new class.

As illustrated by this example, many features referred to in the LAMMPS documentation are called a “style” of a particular command.

The Modify page lists all the common styles in LAMMPS, and discusses the header file for the base class that these styles derive from. Public variables in that file can be used and set by the derived classes, and may also be used by the base class. Sometimes they are also accessed by the rest of LAMMPS. Pure functions, which means functions declared as virtual in the base class header file and which are also set to 0, are functions you must implement in your new derived class to give it the functionality LAMMPS expects. Virtual functions that are not set to 0 are functions you may override or not. Those are usually defined with an empty function body.

Additionally, new output options can be added directly to the thermo.cpp, dump_custom.cpp, and variable.cpp files. These are also listed on the Modify page.

Here are additional guidelines for modifying LAMMPS and adding new functionality:

- Think about whether what you want to do would be better as a pre- or post-processing step. Many computations are more easily and more quickly done that way.

- Do not try to do anything within the timestepping of a run that is not parallel. For example, do not accumulate a bunch of data on a single processor and analyze it. That would run the risk of seriously degrading the parallel efficiency.

- If your new feature reads arguments or writes output, make sure you follow the unit conventions discussed by the units command.

(Foo) Foo, Morefoo, and Maxfoo, J of Classic Potentials, 75, 345 (1997).