# Basic build options

The following topics are covered on this page, for building with both
CMake and make:

-   [Serial vs parallel build](serial)
-   [Choice of compiler and compile/link options](compile)
-   [Build the LAMMPS executable and library](exe)
-   [Including and removing debug support](debug)
-   [Install LAMMPS after a build](install)

------------------------------------------------------------------------

## Serial vs parallel build {#serial}

LAMMPS is written to use the ubiquitous [MPI (Message Passing
Interface)](https://en.wikipedia.org/wiki/Message_Passing_Interface)\_
library API for distributed memory parallel computation. You need to
have such a library installed for building and running LAMMPS in
parallel using a domain decomposition parallelization. It is compatible
with the MPI standard version 2.x and later. LAMMPS can also be built
into a \"serial\" executable for use with a single processor using the
bundled MPI STUBS library.

Independent of the distributed memory MPI parallelization, parts of
LAMMPS are also written with support for shared memory parallelization
using the [OpenMP](https://en.wikipedia.org/wiki/OpenMP)\_ threading
standard. A more detailed discussion of that is below.

::::::: tabs
::: tab
CMake build

``` bash
-D BUILD_MPI=value        # yes or no, default is yes if CMake finds MPI, else no
-D BUILD_OMP=value        # yes or no, default is yes if a compatible compiler is detected
-D LAMMPS_MACHINE=name    # name = mpi, serial, mybox, titan, laptop, etc
                          # no default value
```

The executable created by CMake (after running make) is named `lmp`
unless the `LAMMPS_MACHINE` option is set. When setting
`LAMMPS_MACHINE=name`, the executable will be called `lmp_name`. Using
`BUILD_MPI=no` will enforce building a serial executable using the MPI
STUBS library.
:::

::::: tab
Traditional make

The build with traditional makefiles has to be done inside the source
folder `src`.

``` bash
make mpi                # parallel build, produces lmp_mpi using Makefile.mpi
make serial             # serial build, produces lmp_serial using Makefile/serial
make mybox              # uses Makefile.mybox to produce lmp_mybox
```

Any `make machine` command will look up the make settings from a file
`Makefile.machine` in the folder `src/MAKE` or one of its subdirectories
`MINE`, `MACHINES`, or `OPTIONS`, create a folder `Obj_machine` with all
objects and generated files and an executable called `lmp_machine`. The
standard parallel build with `make mpi` assumes a standard MPI
installation with MPI compiler wrappers where all necessary compiler and
linker flags to get access and link with the suitable MPI headers and
libraries are set by the wrapper programs. For other cases or the serial
build, you have to adjust the make file variables `MPI_INC`, `MPI_PATH`,
`MPI_LIB` as well as `CC` and `LINK`. To enable OpenMP threading usually
a compiler specific flag needs to be added to the compile and link
commands. For the GNU compilers, this is `-fopenmp`, which can be added
to the `CC` and `LINK` makefile variables.

For the serial build the following make variables are set (see
src/MAKE/Makefile.serial):

``` make
CC =            g++
LINK =          g++
MPI_INC =       -I../STUBS
MPI_PATH =      -L../STUBS
MPI_LIB =       -lmpi_stubs
```

You also need to build the STUBS library for your platform before making
LAMMPS itself. A `make serial` build does this for you automatically,
otherwise, type `make mpi-stubs` from the src directory, or `make` from
the `src/STUBS` dir. If the build fails, you may need to edit the
`STUBS/Makefile` for your platform. The stubs library does not provide
MPI/IO functions required by some LAMMPS packages, e.g. `MPIIO` or
`LATBOLTZ`, and thus is not compatible with those packages.

:::: note
::: title
Note
:::

The file `src/STUBS/mpi.cpp` provides a CPU timer function called
`MPI_Wtime()` that calls `gettimeofday()`. If your operating system does
not support `gettimeofday()`, you will need to insert code to call
another timer. Note that the ANSI-standard function `clock()` rolls over
after an hour or so, and is therefore insufficient for timing long
LAMMPS simulations.
::::
:::::
:::::::

### MPI and OpenMP support in LAMMPS

If you are installing MPI yourself to build a parallel LAMMPS
executable, we recommend either MPICH or OpenMPI, which are regularly
used and tested with LAMMPS by the LAMMPS developers. MPICH can be
downloaded from the [MPICH home page](https://www.mpich.org)\_, and
OpenMPI can be downloaded correspondingly from the [OpenMPI home
page](https://www.open-mpi.org)\_. Other MPI packages should also work.
No specific vendor provided and standard compliant MPI library is
currently known to be incompatible with LAMMPS. If you are running on a
large parallel machine, your system admins or the vendor should have
already installed a version of MPI, which is likely to be faster than a
self-installed MPICH or OpenMPI, so you should study the provided
documentation to find out how to build and link with it.

The majority of OpenMP (threading) support in LAMMPS is provided by the
`OPENMP` package; see the [Speed_omp]{.title-ref} page for details. The
`INTEL` package also includes OpenMP threading (it is compatible with
`OPENMP` and will usually fall back on styles from that package, if a
`INTEL` does not exist) and adds vectorization support when compiled
with compatible compilers, in particular the Intel compilers on top of
OpenMP. Also, the `KOKKOS` package can be compiled to include OpenMP
threading.

In addition, there are a few commands in LAMMPS that have native OpenMP
support included as well. These are commands in the `MPIIO`, `ML-SNAP`,
`DIFFRACTION`, and `DPD-REACT` packages. Furthermore, some packages
support OpenMP threading indirectly through the libraries they interface
to: e.g. `KSPACE`, and `COLVARS`. See the [Packages
details](Packages_details) page for more info on these packages, and the
pages for their respective commands for OpenMP threading info.

For CMake, if you use `BUILD_OMP=yes`, you can use these packages and
turn on their native OpenMP support and turn on their native OpenMP
support at run time, by setting the `OMP_NUM_THREADS` environment
variable before you launch LAMMPS.

For building via conventional make, the `CCFLAGS` and `LINKFLAGS`
variables in Makefile.machine need to include the compiler flag that
enables OpenMP. For the GNU compilers or Clang, it is `-fopenmp`. For
(recent) Intel compilers, it is `-qopenmp`. If you are using a different
compiler, please refer to its documentation.

### OpenMP Compiler compatibility {#default-none-issues}

Some compilers do not fully support the `default(none)` directive and
others (e.g. GCC version 9 and beyond, Clang version 10 and later) may
implement strict OpenMP 4.0 and later semantics, which are incompatible
with the OpenMP 3.1 semantics used in LAMMPS for maximal compatibility
with compiler versions in use. If compilation with OpenMP enabled fails
because of your compiler requiring strict OpenMP 4.0 semantics, you can
change the behavior by adding `-D LAMMPS_OMP_COMPAT=4` to the `LMP_INC`
variable in your makefile, or add it to the command line while
configuring with CMake. LAMMPS will auto-detect a suitable setting for
most GNU, Clang, and Intel compilers.

------------------------------------------------------------------------

## Choice of compiler and compile/link options {#compile}

The choice of compiler and compiler flags can be important for maximum
performance. Vendor provided compilers for a specific hardware can
produce faster code than open-source compilers like the GNU compilers.
On the most common x86 hardware, the most popular C++ compilers are
quite similar in their ability to optimize regular C/C++ source code at
high optimization levels. When using the `INTEL` package, there is a
distinct advantage in using the [Intel C++
compiler](https://software.intel.com/en-us/intel-compilers)\_ due to
much improved vectorization through SSE and AVX instructions on
compatible hardware. The source code in that package conditionally
includes compiler specific directives to enable these high degrees of
vectorization. This may change over time as equivalent vectorization
directives are included into the OpenMP standard and other compilers
adopt them.

On parallel clusters or supercomputers which use \"environment modules\"
for their compile/link environments, you can often access different
compilers by simply loading the appropriate module before building
LAMMPS.

::::::::: tabs
::::: tab
CMake build

By default CMake will use the compiler it finds according to internal
preferences, and it will add optimization flags appropriate to that
compiler and any [accelerator packages](Speed_packages) you have
included in the build. CMake will check if the detected or selected
compiler is compatible with the C++ support requirements of LAMMPS and
stop with an error, if this is not the case.

You can tell CMake to look for a specific compiler with setting CMake
variables (listed below) during configuration. For a few common choices,
there are also presets in the `cmake/presets` folder. For convenience,
there is a `CMAKE_TUNE_FLAGS` variable that can be set to apply global
compiler options (applied to compilation only), to be used for adding
compiler or host specific optimization flags in addition to the
\"flags\" variables listed below. You may also specify the corresponding
`CMAKE_*_FLAGS` variables individually, if you want to experiment with
alternate optimization flags. You should specify all 3 compilers, so
that the (few) LAMMPS source files written in C or Fortran are built
with a compiler consistent with the one used for the C++ files:

``` bash
-D CMAKE_CXX_COMPILER=name            # name of C++ compiler
-D CMAKE_C_COMPILER=name              # name of C compiler
-D CMAKE_Fortran_COMPILER=name        # name of Fortran compiler

-D CMAKE_CXX_FLAGS=string             # flags to use with C++ compiler
-D CMAKE_C_FLAGS=string               # flags to use with C compiler
-D CMAKE_Fortran_FLAGS=string         # flags to use with Fortran compiler
```

A few example command lines are:

``` bash
# Building with GNU Compilers:
cmake ../cmake -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++ -DCMAKE_Fortran_COMPILER=gfortran
# Building with Intel Compilers:
cmake ../cmake -DCMAKE_C_COMPILER=icc -DCMAKE_CXX_COMPILER=icpc -DCMAKE_Fortran_COMPILER=ifort
# Building with Intel oneAPI Compilers:
cmake ../cmake -DCMAKE_C_COMPILER=icx -DCMAKE_CXX_COMPILER=icpx -DCMAKE_Fortran_COMPILER=ifx
# Building with LLVM/Clang Compilers:
cmake ../cmake -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_Fortran_COMPILER=flang
# Building with PGI/Nvidia Compilers:
cmake ../cmake -DCMAKE_C_COMPILER=pgcc -DCMAKE_CXX_COMPILER=pgc++ -DCMAKE_Fortran_COMPILER=pgfortran
```

For compiling with the Clang/LLVM compilers a CMake preset is provided
that can be loaded with [-C ../cmake/presets/clang.cmake]{.title-ref}.
Similarly, [-C ../cmake/presets/intel.cmake]{.title-ref} should switch
the compiler toolchain to the legacy Intel compilers, [-C
../cmake/presets/oneapi.cmake]{.title-ref} will switch to the LLVM based
oneAPI Intel compilers, and [-C ../cmake/presets/pgi.cmake]{.title-ref}
will switch the compiler to the PGI compilers.

Furthermore, you can set `CMAKE_TUNE_FLAGS` to specifically add compiler
flags to tune for optimal performance on given hosts. This variable is
empty by default.

:::: note
::: title
Note
:::

When the cmake command completes, it prints a summary to the screen
which compilers it is using and what flags and settings will be used for
the compilation. Note that if the top-level compiler is mpicxx, it is
simply a wrapper on a real compiler. The underlying compiler info is
what CMake will try to determine and report. You should check to confirm
you are using the compiler and optimization flags you want.
::::
:::::

::::: tab
Makefile.machine settings for traditional make

The \"compiler/linker settings\" section of a Makefile.machine lists
compiler and linker settings for your C++ compiler, including
optimization flags. For a parallel build it is recommended to use
`mpicxx` or `mpiCC`, since these compiler wrappers will include a
variety of settings appropriate for your MPI installation and thus
avoiding the guesswork of finding the right flags.

Parallel build (see `src/MAKE/Makefile.mpi`):

``` make
CC =            mpicxx
CCFLAGS =       -g -O3
LINK =          mpicxx
LINKFLAGS =     -g -O
```

Serial build with GNU gcc (see `src/MAKE/Makefile.serial`):

``` make
CC =            g++
CCFLAGS =       -g -O3
LINK =          g++
LINKFLAGS =     -g -O
```

:::: note
::: title
Note
:::

If compilation stops with a message like the following:

``` output
g++ -g -O3  -DLAMMPS_GZIP -DLAMMPS_MEMALIGN=64    -I../STUBS     -c ../main.cpp
In file included from ../pointers.h:24:0,
           from ../input.h:17,
           from ../main.cpp:16:
../lmptype.h:34:2: error: #error LAMMPS requires a C++11 (or later) compliant compiler. Enable C++11 compatibility or upgrade the compiler.
```

then you have either an unsupported (old) compiler or you have to turn
on C++11 mode. The latter applies to GCC 4.8.x shipped with RHEL 7.x and
CentOS 7.x or GCC 5.4.x shipped with Ubuntu16.04. For those compilers,
you need to add the `-std=c++11` flag. If there is no compiler that
supports this flag (or equivalent), you would have to install a newer
compiler that supports C++11; either as a binary package or through
compiling from source.

If you build LAMMPS with any [Speed_packages]{.title-ref} included,
there may be specific compiler or linker flags that are either required
or recommended to enable required features and to achieve optimal
performance. You need to include these in the CCFLAGS and LINKFLAGS
settings above. For details, see the documentation for the individual
packages listed on the [Speed_packages]{.title-ref} page. Or examine
these files in the src/MAKE/OPTIONS directory. They correspond to each
of the 5 accelerator packages and their hardware variants:

``` bash
Makefile.opt                   # OPT package
Makefile.omp                   # OPENMP package
Makefile.intel_cpu             # INTEL package for CPUs
Makefile.intel_coprocessor     # INTEL package for KNLs
Makefile.gpu                   # GPU package
Makefile.kokkos_cuda_mpi       # KOKKOS package for GPUs
Makefile.kokkos_omp            # KOKKOS package for CPUs (OpenMP)
Makefile.kokkos_phi            # KOKKOS package for KNLs (OpenMP)
```
::::
:::::
:::::::::

------------------------------------------------------------------------

## Build the LAMMPS executable and library[]{#exe} {#library}

LAMMPS is always built as a library of C++ classes plus an executable.
The executable is a simple `main()` function that sets up MPI and then
creates a LAMMPS class instance from the LAMMPS library, which will then
process commands provided via a file or from the console input. The
LAMMPS library can also be called from another application or a
scripting language. See the [Howto couple](Howto_couple) doc page for
more info on coupling LAMMPS to other codes. See the
[Python](Python_head) page for more info on wrapping and running LAMMPS
from Python via its library interface.

::::: tabs
::: tab
CMake build

For CMake builds, you can select through setting CMake variables between
building a shared or a static LAMMPS library and what kind of suffix is
added to them (in case you want to concurrently install multiple
variants of binaries with different settings). If none are set, defaults
are applied.

``` bash
-D BUILD_SHARED_LIBS=value   # yes or no (default)
-D LAMMPS_MACHINE=name       # name = mpi, serial, mybox, titan, laptop, etc
                             # no default value
```

The compilation will always produce a LAMMPS library and an executable
linked to it. By default, this will be a static library named
`liblammps.a` and an executable named `lmp` Setting
`BUILD_SHARED_LIBS=yes` will instead produce a shared library called
`liblammps.so` (or `liblammps.dylib` or `liblammps.dll` depending on the
platform) If `LAMMPS_MACHINE=name` is set in addition, the name of the
generated libraries will be changed to either `liblammps_name.a` or
`liblammps_name.so`, respectively and the executable will be called
`lmp_name`.
:::

::: tab
Traditional make

With the traditional makefile based build process, the choice of the
generated executable or library depends on the \"mode\" setting. Several
options are available and `mode=static` is the default.

``` bash
make machine               # build LAMMPS executable lmp_machine
make mode=static machine   # same as "make machine"
make mode=shared machine   # build LAMMPS shared lib liblammps_machine.so instead
```

The \"static\" build will generate a static library called
`liblammps_machine.a` and an executable named `lmp_machine`, while the
\"shared\" build will generate a shared library `liblammps_machine.so`
instead and `lmp_machine` will be linked to it. The build step will also
create generic soft links, named `liblammps.a` and `liblammps.so`, which
point to the specific `liblammps_machine.a/so` files.
:::
:::::

### Additional information

Note that for creating a shared library, all the libraries it depends on
must be compiled to be compatible with shared libraries. This should be
the case for libraries included with LAMMPS, such as the dummy MPI
library in `src/STUBS` or any package libraries in the `lib` directory,
since they are always built in a shared library compatible way using the
`-fPIC` compiler switch. However, if an auxiliary library (like MPI or
FFTW) does not exist as a compatible format, the shared library linking
step may generate an error. This means you will need to install a
compatible version of the auxiliary library. The build instructions for
that library should tell you how to do this.

As an example, here is how to build and install the [MPICH
library](https://www.mpich.org)\_, a popular open-source version of MPI,
as a shared library in the default /usr/local/lib location:

``` bash
./configure --enable-shared
make
make install
```

You may need to use `sudo make install` in place of the last line if you
do not have write privileges for `/usr/local/lib` or use the `--prefix`
configuration option to select an installation folder, where you do have
write access. The end result should be the file
`/usr/local/lib/libmpich.so`. On many Linux installations, the folder
`${HOME}/.local` is an alternative to using `/usr/local` and does not
require superuser or sudo access. In that case the configuration step
becomes:

``` bash
./configure --enable-shared --prefix=${HOME}/.local
```

Avoiding the use of \"sudo\" for custom software installation (i.e. from
source and not through a package manager tool provided by the OS) is
generally recommended to ensure the integrity of the system software
installation.

------------------------------------------------------------------------

## Including or removing debug support {#debug}

By default the compilation settings will include the *-g* flag which
instructs the compiler to include debug information (e.g. which line of
source code a particular instruction correspond to). This can be
extremely useful in case LAMMPS crashes and can help to provide crucial
information in [tracking down the origin of a crash](Errors_debug) and
help the LAMMPS developers fix bugs in the source code. However, this
increases the storage requirements for object files, libraries, and the
executable 3-5 fold.

If this is a concern, you can change the compilation settings or remove
the debug information from the LAMMPS executable:

-   **Traditional make**: edit your `Makefile.<machine>` to remove the
    *-g* flag from the `CCFLAGS` and `LINKFLAGS` definitions
-   **CMake**: use `-D CMAKE_BUILD_TYPE=Release` or explicitly reset the
    applicable compiler flags (best done using the text mode or
    graphical user interface).
-   **Remove debug info**: If you are only concerned about the
    executable being too large, you can use the `strip` tool (e.g.
    `strip lmp_serial`) to remove the debug information from the
    executable file. Do not strip libraries or object files, as that
    will render them unusable.

------------------------------------------------------------------------

## Build LAMMPS tools {#tools}

Some tools described in [Auxiliary tools](Tools) can be built directly
using CMake or Make.

::::: tabs
::: tab
CMake build

``` bash
-D BUILD_TOOLS=value         # yes or no (default)
-D BUILD_LAMMPS_SHELL=value  # yes or no (default)
```

The generated binaries will also become part of the LAMMPS installation
(see below).
:::

::: tab
Traditional make

``` bash
cd lammps/tools
make all              # build all binaries of tools
make binary2txt       # build only binary2txt tool
make chain            # build only chain tool
make micelle2d        # build only micelle2d tool
make thermo_extract   # build only thermo_extract tool

cd lammps/tools/lammps-shell
make                  # build LAMMPS shell
```
:::
:::::

------------------------------------------------------------------------

## Install LAMMPS after a build {#install}

After building LAMMPS, you may wish to copy the LAMMPS executable or
library, along with other LAMMPS files (library header, doc files), to a
globally visible place on your system, for others to access. Note that
you may need super-user privileges (e.g. sudo) if the directory you want
to copy files to is protected.

::::: tabs
::: tab
CMake build

``` bash
cmake -D CMAKE_INSTALL_PREFIX=path [options ...] ../cmake
make                        # perform make after CMake command
make install                # perform the installation into prefix
```

During the installation process CMake will by default remove any runtime
path settings for loading shared libraries. Because of this you may have
to set or modify the `LD_LIBRARY_PATH` (or `DYLD_LIBRARY_PATH`)
environment variable, if you are installing LAMMPS into a non-system
location and/or are linking to libraries in a non-system location that
depend on such runtime path settings. As an alternative, you may set the
CMake variable `LAMMPS_INSTALL_RPATH` to `on` and then the runtime paths
for any linked shared libraries and the library installation folder for
the LAMMPS library will be embedded and thus the requirement to set
environment variables is avoided. The `off` setting is usually preferred
for packaged binaries or when setting up environment modules, the `on`
setting is more convenient for installing software into a non-system or
personal folder.
:::

::: tab
Traditional make

There is no \"install\" option in the `src/Makefile` for LAMMPS. If you
wish to do this you will need to first build LAMMPS, then manually copy
the desired LAMMPS files to the appropriate system directories.
:::
:::::