# LAMMPS input scripts

LAMMPS executes calculations by reading commands from a input script
(text file), one line at a time. When the input script ends, LAMMPS
exits. This is different from programs that read and process the entire
input before starting a calculation.

Each command causes LAMMPS to take some immediate action without regard
for any commands that may be processed later. Commands may set an
internal variable, read in a file, or run a simulation. These actions
can be grouped into three categories:

a)  commands that change a global setting (examples: timestep, newton,
    echo, log, thermo, restart),
b)  commands that add, modify, remove, or replace \"styles\" that are
    executed during a \"run\" (examples: pair_style, fix, compute, dump,
    thermo_style, pair_modify), and
c)  commands that execute a \"run\" or perform some other computation or
    operation (examples: print, run, minimize, temper, write_dump,
    rerun, read_data, read_restart)

Commands in category a) have default settings, which means you only need
to use the command if you wish to change the defaults.

In many cases, the ordering of commands in an input script is not
important, but can have consequences when the global state is changed
between commands in the c) category. The following rules apply:

(1) LAMMPS does not read your entire input script and then perform a
    simulation with all the settings. Rather, the input script is read
    one line at a time and each command takes effect when it is read.
    Thus this sequence of commands:

    ``` LAMMPS
    timestep 0.5
    run      100
    run      100
    ```

    does something different than this sequence:

    ``` LAMMPS
    run      100
    timestep 0.5
    run      100
    ```

    In the first case, the specified timestep (0.5 fs) is used for two
    simulations of 100 timesteps each. In the second case, the default
    timestep (1.0 fs) is used for the first 100 step simulation and a
    0.5 fs timestep is used for the second one.

(2) Some commands are only valid when they follow other commands. For
    example you cannot set the temperature of a group of atoms until
    atoms have been defined and a group command is used to define which
    atoms belong to the group.

(3) Sometimes command B will use values that can be set by command A.
    This means command A must precede command B in the input script if
    it is to have the desired effect. For example, the
    [read_data](read_data) command initializes the system by setting up
    the simulation box and assigning atoms to processors. If default
    values are not desired, the [processors](processors) and
    [boundary](boundary) commands need to be used before read_data to
    tell LAMMPS how to map processors to the simulation box.

Many input script errors are detected by LAMMPS and an ERROR or WARNING
message is printed. The [Errors](Errors) page gives more information on
what errors mean. The documentation for each command lists restrictions
on how the command can be used.

You can use the [-skiprun](skiprun) command line flag to have LAMMPS
skip the execution of any \"run\", \"minimize\", or similar commands to
check the entire input for correct syntax to avoid crashes on typos or
syntax errors in long runs.