Refer to "mixed" precision in user-facing documentation

[gromacs.git] / install-guide / install-guide.md

% Installation guide for GROMACS @PROJECT_VERSION@

# Building GROMACS #

These instructions pertain to building GROMACS
@PROJECT_VERSION@. Up-to-date installation instructions may be found
at <http://www.gromacs.org/Documentation/Installation_Instructions>.

# Quick and dirty installation #

1. Get the latest version of your C and C++ compilers.
2. Check that you have CMake version @GMX_CMAKE_MINIMUM_REQUIRED_VERSION@ or later.
3. Get and unpack the latest version of the GROMACS tarball.
4. Make a separate build directory and change to it. 
5. Run `cmake` with the path to the source as an argument
6. Run `make` and `make install`

Or, as a sequence of commands to execute:

    tar xfz gromacs-@PROJECT_VERSION@.tar.gz
    cd gromacs-@PROJECT_VERSION@
    mkdir build
    cd build
    cmake .. -DGMX_BUILD_OWN_FFTW=ON
    make
    sudo make install
    source /usr/local/gromacs/bin/GMXRC

This will download and build first the prerequisite FFT library
followed by GROMACS. If you already have FFTW installed, you can
remove that argument to `cmake`. Overall, this build of GROMACS will
be correct and reasonably fast on the machine upon which `cmake`
ran. If you want to get the maximum value for your hardware with
GROMACS, you will have to read further. Sadly, the interactions of
hardware, libraries, and compilers are only going to continue to get
more complex.

# Typical GROMACS installation #

As above, and with further details below, but you should consider
using the following [CMake options](#using-cmake-command-line-options) with the
appropriate value instead of `xxx` :

* `-DCMAKE_C_COMPILER=xxx` equal to the name of the C99 [compiler](#compiler) you wish to use (or the environment variable `CC`)
* `-DCMAKE_CXX_COMPILER=xxx` equal to the name of the C++98 [compiler](#compiler) you wish to use (or the environment variable `CXX`)
* `-DGMX_MPI=on` to build using an [MPI](#mpi-support) wrapper compiler
* `-DGMX_GPU=on` to build using nvcc to run with an NVIDIA [GPU](#native-gpu-acceleration)
* `-DGMX_SIMD=xxx` to specify the level of [SIMD support](#simd-support) of the node on which `mdrun` will run
* `-DGMX_BUILD_MDRUN_ONLY=on` to [build only the `mdrun` binary](#building-only-mdrun), e.g. for compute cluster back-end nodes
* `-DGMX_DOUBLE=on` to run GROMACS in double precision (slower, and not normally useful)
* `-DCMAKE_PREFIX_PATH=xxx` to add a non-standard location for CMake to [search for libraries](#helping-cmake-find-the-right-librariesheadersprograms)
* `-DCMAKE_INSTALL_PREFIX=xxx` to install GROMACS to a non-standard location (default `/usr/local/gromacs`)
* `-DBUILD_SHARED_LIBS=off` to turn off the building of [shared libraries](#static-linking)
* `-DGMX_FFT_LIBRARY=xxx` to select whether to use `fftw`, `mkl` or `fftpack` libraries for [FFT support](#fast-fourier-transform-library)
* `-DCMAKE_BUILD_TYPE=Debug` to build GROMACS in debug mode

# Building older GROMACS versions #

For installation instructions for old GROMACS versions, see the
documentation at
<http://www.gromacs.org/Documentation/Installation_Instructions_4.5>
and
<http://www.gromacs.org/Documentation/Installation_Instructions_4.6>

# Prerequisites #

## Platform ##

GROMACS can be compiled for many operating systems and architectures.
These include any distribution of Linux, Mac OS X or Windows, and
architectures including x86, AMD64/x86-64, PPC, ARM v7 and SPARC VIII.

## Compiler ##

Technically, GROMACS can be compiled on any platform with an ANSI C99
and C++98 compiler, and their respective standard C/C++ libraries.
Getting good performance on an OS and architecture requires choosing a
good compiler. In practice, many compilers struggle to do a good job
optimizing the GROMACS architecture-optimized SIMD kernels.

For best performance, the GROMACS team strongly recommends you get the
most recent version of your preferred compiler for your platform.
There is a large amount of GROMACS code that depends on effective
compiler optimization to get high performance. This makes GROMACS
performance sensitive to the compiler used, and the binary will often
only work on the hardware for which it is compiled.

* In particular, GROMACS includes a lot of explicit SIMD
(single instruction, multiple data) optimization that can use
assembly instructions available on most modern processors. This
can have a substantial effect on performance, but for recent
processors you also need a similarly recent compiler that includes
support for the corresponding SIMD instruction set to get this
benefit. The configuration does a good job at detecting this,
and you will usually get warnings if GROMACS and your hardware
support a more recent instruction set than your compiler.

* On Intel-based x86 hardware, we recommend you to use the GNU
compilers version 4.7 or later or Intel compilers version 12 or later
for best performance. The Intel compiler has historically been better
at instruction scheduling, but recent gcc versions have proved to be
as fast or sometimes faster than Intel.

* The Intel and GNU compilers produce much faster GROMACS executables
than the PGI and Cray compilers.

* On AMD-based x86 hardware up through the "K10" microarchitecture
("Family 10h") Thuban/Magny-Cours architecture (e.g. Opteron
6100-series processors), it is worth using the Intel compiler for
better performance, but gcc version 4.7 and later are also reasonable.

* On the AMD Bulldozer architecture (Opteron 6200), AMD introduced
fused multiply-add instructions and an "FMA4" instruction format not
available on Intel x86 processors. Thus, on the most recent AMD
processors you want to use gcc version 4.7 or later for best
performance! The Intel compiler will only generate code for the subset
also supported by Intel processors, and that is significantly slower.

* If you are running on Mac OS X, the best option is the Intel
compiler. Both clang and gcc will work, but they produce lower
performance and each have some shortcomings. Current Clang does not
support OpenMP. This may change when clang 3.5 becomes available.

* For all non-x86 platforms, your best option is typically to use the
vendor's default or recommended compiler, and check for specialized
information below.

## Compiling with parallelization options ##

GROMACS can run in parallel on multiple cores of a single
workstation using its built-in thread-MPI. No user action is required
in order to enable this.

### GPU support ###

If you wish to use the excellent native GPU support in GROMACS,
NVIDIA's [CUDA](http://www.nvidia.com/object/cuda_home_new.html)
version @REQUIRED_CUDA_VERSION@ software development kit is required,
and the latest version is strongly encouraged. NVIDIA GPUs with at
least NVIDIA compute capability @REQUIRED_CUDA_COMPUTE_CAPABILITY@ are
required, e.g. Fermi or Kepler cards. You are strongly recommended to
get the latest CUDA version and driver supported by your hardware, but
beware of possible performance regressions in newer CUDA versions on
older hardware. Note that while some CUDA compilers (nvcc) might not
officially support recent versions of gcc as the back-end compiler, we
still recommend that you at least use a gcc version recent enough to
get the best SIMD support for your CPU, since GROMACS always runs some
code on the CPU. It is most reliable to use the same C++ compiler
version for GROMACS code as used as the back-end compiler for nvcc,
but it could be faster to mix compiler versions to suit particular
contexts.

### MPI support ###

If you wish to run in parallel on multiple machines across a network,
you will need to have

* an MPI library installed that supports the MPI 1.3
  standard, and
* wrapper compilers that will compile code using that library.

The GROMACS team recommends [OpenMPI](http://www.open-mpi.org) version
1.6 (or higher), [MPICH](http://www.mpich.org) version 1.4.1 (or
higher), or your hardware vendor's MPI installation. The most recent
version of either of these is likely to be the best. More specialized
networks might depend on accelerations only available in the vendor's
library. [LAMMPI](http://www.lam-mpi.org) might work, but since it has
been deprecated for years, it is not supported.

Often [OpenMP](http://en.wikipedia.org/wiki/OpenMP) parallelism is an
advantage for GROMACS, but support for this is generally built into
your compiler and detected automatically.

In summary, for maximum performance you will need to examine how you
will use GROMACS, what hardware you plan to run on, and whether you
can afford a non-free compiler for slightly better
performance. Unfortunately, the only way to find out is to test
different options and parallelization schemes for the actual
simulations you want to run. You will still get *good*,
performance with the default build and runtime options, but if you
truly want to push your hardware to the performance limit, the days of
just blindly starting programs with `mdrun` are gone.

## CMake ##

GROMACS @PROJECT_VERSION@ uses the CMake build system, and requires
version @GMX_CMAKE_MINIMUM_REQUIRED_VERSION@ or higher. Lower versions
will not work. You can check whether CMake is installed, and what
version it is, with `cmake --version`. If you need to install CMake,
then first check whether your platform's package management system
provides a suitable version, or visit
<http://www.cmake.org/cmake/help/install.html> for pre-compiled
binaries, source code and installation instructions. The GROMACS team
recommends you install the most recent version of CMake you can.

## Fast Fourier Transform library ##

Many simulations in GROMACS make extensive use of fast Fourier
transforms, and a software library to perform these is always
required. We recommend [FFTW](http://www.fftw.org) (version 3 or
higher only) or
[Intel MKL](http://software.intel.com/en-us/intel-mkl). The choice of
library can be set with `cmake -DGMX_FFT_LIBRARY=<name>`, where
`<name>` is one of `fftw`, `mkl`, or `fftpack`. FFTPACK is bundled
with GROMACS as a fallback, and is acceptable if mdrun performance is
not a priority.

### FFTW ###

FFTW is likely to be available for your platform via its package
management system, but there can be compatibility and significant
performance issues associated with these packages. In particular,
GROMACS simulations are normally run in "mixed" floating-point
precision, which is suited for the use of single precision in
FFTW. The default FFTW package is normally in double
precision, and good compiler options to use for FFTW when linked to
GROMACS may not have been used. Accordingly, the GROMACS team
recommends either

* that you permit the GROMACS installation to download and
  build FFTW from source automatically for you (use
  `cmake -DGMX_BUILD_OWN_FFTW=ON`), or
* that you build FFTW from the source code.

Note that the GROMACS-managed download of the FFTW tarball has a
slight chance of posing a security risk. If you use this option, you
will see a warning that advises how you can eliminate this risk
(before the opportunity has arisen).

If you build FFTW from source yourself, get the most recent version
and follow its [installation
guide](http://www.fftw.org/doc/Installation-and-Customization.html#Installation-and-Customization).
Choose the precision for FFTW (i.e. single or float vs. double) to
match whether you will later use mixed or double precision for
GROMACS. There is no need to compile FFTW with
threading or MPI support, but it does no harm. On x86 hardware,
compile *only* with `--enable-sse2` (regardless of precision) even if
your processors can take advantage of AVX extensions. Since GROMACS
uses fairly short transform lengths we do not benefit from the FFTW
AVX acceleration, and because of memory system performance
limitations, it can even degrade GROMACS performance by around
20%. There is no way for GROMACS to limit the use to SSE2 SIMD at run
time if AVX support has been compiled into FFTW, so you need to set
this at compile time.

### MKL ###

Using MKL with the Intel Compilers version 11 or higher is very
simple. Set up your compiler environment correctly, perhaps with a
command like `source /path/to/compilervars.sh intel64` (or consult
your local documentation). Then set `-DGMX_FFT_LIBRARY=mkl` when you
run cmake. In this case, GROMACS will also use MKL for BLAS and LAPACK
(see
[linear algebra libraries](#linear-algebra-libraries)). Generally,
there is no advantage in using MKL with GROMACS, and FFTW is often
faster.

Otherwise, you can get your hands dirty and configure MKL by setting

    -DGMX_FFT_LIBRARY=mkl
    -DMKL_LIBRARIES="/full/path/to/libone.so;/full/path/to/libtwo.so"
    -DMKL_INCLUDE_DIR="/full/path/to/mkl/include"

where the full list (and order!) of libraries you require are found in
Intel's MKL documentation for your system.

## Optional build components ##

* Compiling to run on NVIDIA GPUs requires CUDA
* An external Boost library can be used to provide better
  implementation support for smart pointers and exception handling,
  but the GROMACS source bundles a subset of Boost 1.55.0 as a fallback
* Hardware-optimized BLAS and LAPACK libraries are useful
  for a few of the GROMACS utilities focused on normal modes and
  matrix manipulation, but they do not provide any benefits for normal
  simulations. Configuring these are discussed at
  [linear algebra libraries](#linear-algebra-libraries).
* The built-in GROMACS trajectory viewer `gmx view` requires X11 and
  Motif/Lesstif libraries and header files. You may prefer to use
  third-party software for visualization, such as
  [VMD](http://www.ks.uiuc.edu/Research/vmd) or
  [PyMOL](http://www.pymol.org).
* An external TNG library for trajectory-file handling can be used,
  but TNG 1.6 is bundled in the GROMACS source already
* zlib is used by TNG for compressing some kinds of trajectory data
* Running the GROMACS test suite requires libxml2
* Building the GROMACS documentation requires ImageMagick, pdflatex,
  bibtex, doxygen and pandoc.
* The GROMACS utility programs often write data files in formats
  suitable for the Grace plotting tool, but it is straightforward to
  use these files in other plotting programs, too.

# Doing a build of GROMACS #

This section will cover a general build of GROMACS with CMake, but it
is not an exhaustive discussion of how to use CMake. There are many
resources available on the web, which we suggest you search for when
you encounter problems not covered here. The material below applies
specifically to builds on Unix-like systems, including Linux, and Mac
OS X. For other platforms, see the specialist instructions below.

## Configuring with CMake ##

CMake will run many tests on your system and do its best to work out
how to build GROMACS for you. If your build machine is the same as
your target machine, then you can be sure that the defaults will be
pretty good. The build configuration will for instance attempt to
detect the specific hardware instructions available in your
processor. However, if you want to control aspects of the build, or
you are compiling on a cluster head node for back-end nodes with a
different architecture, there are plenty of things you can set
manually.

The best way to use CMake to configure GROMACS is to do an
"out-of-source" build, by making another directory from which you will
run CMake. This can be outside the source directory, or a subdirectory
of it. It also means you can never corrupt your source code by trying
to build it! So, the only required argument on the CMake command line
is the name of the directory containing the `CMakeLists.txt` file of
the code you want to build. For example, download the source tarball
and use

    $ tar xfz gromacs-@PROJECT_VERSION@.tgz
    $ cd gromacs-@PROJECT_VERSION@
    $ mkdir build-gromacs
    $ cd build-gromacs
    $ cmake ..

You will see `cmake` report a sequence of results of tests and
detections done by the GROMACS build system. These are written to the
`cmake` cache, kept in `CMakeCache.txt`. You can edit this file by
hand, but this is not recommended because you could make a mistake.
You should not attempt to move or copy this file to do another build,
because file paths are hard-coded within it. If you mess things up,
just delete this file and start again with `cmake`.

If there is a serious problem detected at this stage, then you will see
a fatal error and some suggestions for how to overcome it. If you are
not sure how to deal with that, please start by searching on the web
(most computer problems already have known solutions!) and then
consult the gmx-users mailing list. There are also informational
warnings that you might like to take on board or not. Piping the
output of `cmake` through `less` or `tee` can be
useful, too.

Once `cmake` returns, you can see all the settings that were chosen
and information about them by using e.g. the curses interface

    $ ccmake ..

You can actually use `ccmake` (available on most Unix platforms,
if the curses library is supported) directly in the first step, but then
most of the status messages will merely blink in the lower part
of the terminal rather than be written to standard out. Most platforms
including Linux, Windows, and Mac OS X even have native graphical user interfaces for
`cmake`, and it can create project files for almost any build environment
you want (including Visual Studio or Xcode).
Check out <http://www.cmake.org/cmake/help/runningcmake.html> for
general advice on what you are seeing and how to navigate and change
things. The settings you might normally want to change are already
presented. You may make changes, then re-configure (using `c`), so that it
gets a chance to make changes that depend on yours and perform more
checking. This might require several configuration stages when you are
using `ccmake` - when you are using `cmake` the
iteration is done behind the scenes.

A key thing to consider here is the setting of
`CMAKE_INSTALL_PREFIX`. You will need to be able to write to this
directory in order to install GROMACS later, and if you change your
mind later, changing it in the cache triggers a full re-build,
unfortunately. So if you do not have super-user privileges on your
machine, then you will need to choose a sensible location within your
home directory for your GROMACS installation. Even if you do have
super-user privileges, you should use them only for the installation
phase, and never for configuring, building, or running GROMACS!

When `cmake` or `ccmake` have completed iterating, the
cache is stable and a build tree can be generated, with `g` in
`ccmake` or automatically with `cmake`.

You cannot attempt to change compilers after the initial run of
`cmake`. If you need to change, clean up, and start again.

### Using CMake command-line options ###

Once you become comfortable with setting and changing options, you may
know in advance how you will configure GROMACS. If so, you can speed
things up by invoking `cmake` and passing the various options at once
on the command line. This can be done by setting cache variable at the
cmake invocation using the `-DOPTION=VALUE`; note that some
environment variables are also taken into account, in particular
variables like CC, CXX, FCC (which may be familiar to autoconf users).

For example, the following command line

    $ cmake .. -DGMX_GPU=ON -DGMX_MPI=ON -DCMAKE_INSTALL_PREFIX=/home/marydoe/programs

can be used to build with GPUs, MPI and install in a custom
location. You can even save that in a shell script to make it even
easier next time. You can also do this kind of thing with `ccmake`,
but you should avoid this, because the options set with `-D` will not
be able to be changed interactively in that run of `ccmake`.

### SIMD support ###

GROMACS has extensive support for detecting and using the SIMD
capabilities of many modern HPC CPU architectures. If you are building
GROMACS on the same hardware you will run it on, then you don't need
to read more about this, unless you are getting configuration warnings
you do not understand. By default, the GROMACS build system will
detect the SIMD instruction set supported by the CPU architecture (on
which the configuring is done), and thus pick the best
available SIMD parallelization supported by GROMACS. The build system
will also check that the compiler and linker used also support the
selected SIMD instruction set and issue a fatal error if they
do not.

Valid values are listed below, and the
applicable value lowest on the list is generally the one you should
choose:

1. `None` For use only on an architecture either lacking SIMD,
  or to which GROMACS has not yet been ported and none of the
  options below are applicable.
2. `SSE2` This SIMD instruction set was introduced in Intel
   processors in 2001, and AMD in 2003. Essentially all x86
   machines in existence have this, so it might be a good choice if
   you need to support dinosaur x86 computers too.
3. `SSE4.1` Present in all Intel core processors since 2007,
   but notably not in AMD magny-cours. Still, almost all recent
   processors support this, so this can also be considered a good
   baseline if you are content with portability between reasonably
   modern processors.
4. `AVX_128_FMA` AMD bulldozer processors (2011) have this.
   Unfortunately Intel and AMD have diverged the last few years;
   If you want good performance on modern AMD processors
   you have to use this since it also allows the reset of the
   code to use AMD 4-way fused multiply-add instructions. The drawback
   is that your code will not run on Intel processors at all.
5. `AVX_256` This instruction set is present on Intel processors
   since Sandy Bridge (2011), where it is the best choice unless
   you have an even more recent CPU that supports AVX2. While this
   code will work on recent AMD processors, it is significantly
   less efficient than the AVX_128_FMA choice above - do not be
   fooled to assume that 256 is better than 128 in this case.
6. `AVX2_256` Present on Intel Haswell processors released in 2013,
   and it will also enable Intel 3-way fused multiply-add instructions.
   This code will not work on AMD CPUs.
7. `IBM_QPX ` BlueGene/Q A2 cores have this.
8. `Sparc64_HPC_ACE` Fujitsu machines like the K computer have this.

The CMake configure system will check that the compiler you have
chosen can target the architecture you have chosen. `mdrun` will check
further at runtime, so if in doubt, choose the lowest setting you
think might work, and see what `mdrun` says. The configure system also
works around many known issues in many versions of common HPC
compilers. However, since the options also enable general compiler
flags for the platform in question, you can end up in situations
where e.g. an `AVX_128_FMA` binary will just crash on any
Intel machine, since the code will try to execute general illegal
instructions (inserted by the compiler) before `mdrun` gets to the
architecture detection routines.

A further `GMX_SIMD=Reference` option exists, which is a special
SIMD-like implementation written in plain C that developers can use
when developing support in GROMACS for new SIMD architectures. It is
not designed for use in production simulations, but if you are using
an architecture with SIMD support to which GROMACS has not yet been
ported, you may wish to try this option instead of the default
`GMX_SIMD=None`, as it can often out-perform this when the
auto-vectorization in your compiler does a good job. And post on the
GROMACS mailing lists, because GROMACS can probably be ported for new
SIMD architectures in a few days.

### CMake advanced options ###

The options that are displayed in the default view of `ccmake` are
ones that we think a reasonable number of users might want to consider
changing. There are a lot more options available, which you can see by
toggling the advanced mode in `ccmake` on and off with `t`. Even
there, most of the variables that you might want to change have a
`CMAKE_` or `GMX_` prefix. There are also some options that will be
visible or not according to whether their preconditions are satisfied.

### Helping CMake find the right libraries/headers/programs ###

If libraries are installed in non-default locations their location can
be specified using the following environment variables:

* `CMAKE_INCLUDE_PATH` for header files
* `CMAKE_LIBRARY_PATH` for libraries
* `CMAKE_PREFIX_PATH` for header, libraries and binaries
  (e.g. `/usr/local`).

The respective `include`, `lib`, or `bin` is
appended to the path. For each of these variables, a list of paths can
be specified (on Unix, separated with ":"). Note that these are
enviroment variables (and not `cmake` command-line arguments) and in
a `bash` shell are used like:

    $ CMAKE_PREFIX_PATH=/opt/fftw:/opt/cuda cmake ..

Alternatively, these variables are also `cmake` options, so they can
be set like `-DCMAKE_PREFIX_PATH=/opt/fftw:/opt/cuda`.

The `CC` and `CXX` environment variables are also useful
for indicating to `cmake` which compilers to use, which can be very
important for maximising GROMACS performance. Similarly,
`CFLAGS`/`CXXFLAGS` can be used to pass compiler
options, but note that these will be appended to those set by
GROMACS for your build platform and build type. You can customize
some of this with advanced options such as `CMAKE_C_FLAGS`
and its relatives.

See also: <http://cmake.org/Wiki/CMake_Useful_Variables#Environment_Variables>

### Native GPU acceleration ###
If you have the CUDA Toolkit installed, you can use `cmake` with:

    $ cmake .. -DGMX_GPU=ON -DCUDA_TOOLKIT_ROOT_DIR=/usr/local/cuda

(or whichever path has your installation). In some cases, you might
need to specify manually which of your C++ compilers should be used,
e.g. with the advanced option `CUDA_HOST_COMPILER`.

The GPU acceleration has been tested on AMD64/x86-64 platforms with
Linux, Mac OS X and Windows operating systems, but Linux is the
best-tested and supported of these. Linux running on ARM v7 (32 bit)
CPUs also works.

### Static linking ###
Dynamic linking of the GROMACS executables will lead to a
smaller disk footprint when installed, and so is the default on
platforms where we believe it has been tested repeatedly and found to work.
In general, this includes Linux, Windows, Mac OS X and BSD systems.
Static binaries take much more space, but on some hardware and/or under
some conditions they are necessary, most commonly when you are running a parallel
simulation using MPI libraries (e.g. BlueGene, Cray).

* To link GROMACS binaries
statically against the internal GROMACS libraries, set
`-DBUILD_SHARED_LIBS=OFF`.
* To link statically against external (non-system) libraries as well,
the `-DGMX_PREFER_STATIC_LIBS=ON` option can be used. Note, that in
general `cmake` picks up whatever is available, so this option only
instructs `cmake` to prefer static libraries when both static and
shared are available. If no static version of an external library is
available, even when the aforementioned option is `ON`, the shared
library will be used. Also note, that the resulting binaries will
still be dynamically linked against system libraries on platforms
where that is the default. To use static system libraries, additional
compiler/linker flags are necessary, e.g. `-static-libgcc
-static-libstdc++`.

### Portability aspects ###

Here, we consider portability aspects related to CPU instruction sets,
for details on other topics like binaries with statical vs dynamic
linking please consult the relevant parts of this documentation or
other non-GROMACS specific resources.

A GROMACS build will normally not be portable, not even across
hardware with the same base instruction set like x86. Non-portable
hardware-specific optimizations are selected at configure-time, such
as the SIMD instruction set used in the compute-kernels. This
selection will be done by the build system based on the capabilities
of the build host machine or based on cross-compilation information
provided to `cmake` at configuration.

Often it is possible to ensure portability by choosing the least
common denominator of SIMD support, e.g. SSE2 for x86, and ensuring
the you use `cmake -DGMX_USE_RDTSCP=off` if any of the target CPU
architectures does not support the `RDTSCP` instruction.  However, we
discourage attempts to use a single GROMACS installation when the
execution environment is heterogeneous, such as a mix of AVX and
earlier hardware, because this will lead to programs (especially
`mdrun`) that run slowly on the new hardware. Building two full
installations and locally managing how to call the correct one
(e.g. using the module system) is the recommended
approach. Alternatively, as at the moment the GROMACS tools do not
make strong use of SIMD acceleration, it can be convenient to create
an installation with tools portable across different x86 machines, but
with separate `mdrun` binaries for each architecture. To achieve this,
one can first build a full installation with the
least-common-denominator SIMD instruction set, e.g. `-DGMX_SIMD=SSE2`,
then build separate `mdrun` binaries for each architecture present in
the heterogeneous environment. By using custom binary and library
suffixes for the `mdrun`-only builds, these can be installed to the
same location as the "generic" tools installation. Building [only the
`mdrun` binary](#building-only-mdrun) is possible by setting the `-DGMX_BUILD_MDRUN_ONLY=ON`
option.

### Linear algebra libraries ###

As mentioned above, sometimes vendor BLAS and LAPACK libraries
can provide performance enhancements for GROMACS when doing
normal-mode analysis or covariance analysis. For simplicity, the text
below will refer only to BLAS, but the same options are available
for LAPACK. By default, CMake will search for BLAS, use it if it
is found, and otherwise fall back on a version of BLAS internal to
GROMACS. The `cmake` option `-DGMX_EXTERNAL_BLAS=on` will be set
accordingly. The internal versions are fine for normal use. If you
need to specify a non-standard path to search, use
`-DCMAKE_PREFIX_PATH=/path/to/search`. If you need to specify a
library with a non-standard name (e.g. ESSL on AIX or BlueGene), then
set `-DGMX_BLAS_USER=/path/to/reach/lib/libwhatever.a`.

If you are using Intel MKL for FFT, then the BLAS and
LAPACK it provides are used automatically. This could be
over-ridden with `GMX_BLAS_USER`, etc.

On Apple platforms where the Accelerate Framework is available, these
will be automatically used for BLAS and LAPACK. This could be
over-ridden with `GMX_BLAS_USER`, etc.

### Changing the names of GROMACS binaries and libraries ###

It is sometimes convenient to have different versions of the same
GROMACS programs installed. The most common use cases have been single
and double precision, and with and without MPI. This mechanism can
also be used to install side-by-side multiple versions of `mdrun`
optimized for different CPU architectures, as mentioned previously.

By default, GROMACS will suffix programs and libraries for such builds
with `_d` for double precision and/or `_mpi` for MPI (and nothing
otherwise). This can be controlled manually with `GMX_DEFAULT_SUFFIX
(ON/OFF)`, `GMX_BINARY_SUFFIX` (takes a string) and `GMX_LIBS_SUFFIX`
(also takes a string). For instance, to set a custom suffix for
programs and libraries, one might specify:

    cmake .. -DGMX_DEFAULT_SUFFIX=OFF -DGMX_BINARY_SUFFIX=_mod -DGMX_LIBS_SUFFIX=_mod

Thus the names of all programs and libraries will be appended with
`_mod`.

## Building GROMACS ##

Once you have configured with `cmake`, you can build GROMACS. It is
expected that the `make` procedure will always complete successfully,
and give few or no warnings. The tests GROMACS makes on the settings
you choose are pretty extensive, but there are probably a few cases we
have not thought of yet. Search the web first for solutions to
problems, but if you need help, ask on gmx-users, being sure to
provide as much information as possible about what you did, the system
you are building on, and what went wrong. This may mean scrolling back
a long way through the output of `make` to find the first error
message!

If you have a multi-core or multi-CPU machine with `N`
processors, then using
    $ make -j N
will generally speed things up by quite a bit. Other build generator systems
supported by `cmake` (e.g. `ninja`) also work well.

### Building only mdrun ###

Past versions of the build system offered "mdrun" and "install-mdrun"
targets (similarly for other programs too) to build and install only
the mdrun program, respectively. Such a build is useful when the
configuration is only relevant for `mdrun` (such as with
parallelization options for MPI, SIMD, GPUs, or on BlueGene or Cray),
or the length of time for the compile-link-install cycle is relevant
when developing.

This is now supported with the `cmake` option
`-DGMX_BUILD_MDRUN_ONLY=ON`, which will build a cut-down version of
`libgromacs` and/or the `mdrun` program (according to whether shared
or static). Naturally, now `make install` installs only those
products. By default, mdrun-only builds will default to static linking
against GROMACS libraries, because this is generally a good idea for
the targets for which an mdrun-only build is desirable. If you re-use
a build tree and change to the mdrun-only build, then you will inherit
the setting for `BUILD_SHARED_LIBS` from the old build, and will be
warned that you may wish to manage `BUILD_SHARED_LIBS` yourself.

## Installing GROMACS ##

Finally, `make install` will install GROMACS in the
directory given in `CMAKE_INSTALL_PREFIX`. If this is a system
directory, then you will need permission to write there, and you
should use super-user privileges only for `make install` and
not the whole procedure.

## Getting access to GROMACS after installation ##

GROMACS installs the script `GMXRC` in the `bin`
subdirectory of the installation directory
(e.g. `/usr/local/gromacs/bin/GMXRC`), which you should source
from your shell:

    $ source /your/installation/prefix/here/bin/GMXRC

It will detect what kind of shell you are running and set up your
environment for using GROMACS. You may wish to arrange for your
login scripts to do this automatically; please search the web for
instructions on how to do this for your shell. 

Many of the GROMACS programs rely on data installed in the
`share/gromacs` subdirectory of the installation directory. By
default, the programs will use the environment variables set in the
`GMXRC` script, and if this is not available they will try to guess the
path based on their own location.  This usually works well unless you
change the names of directories inside the install tree. If you still
need to do that, you might want to recompile with the new install
location properly set, or edit the `GMXRC` script.

## Testing GROMACS for correctness ##

Since 2011, the GROMACS development uses an automated system where
every new code change is subject to regression testing on a number of
platforms and software combinations. While this improves
reliability quite a lot, not everything is tested, and since we
increasingly rely on cutting edge compiler features there is
non-negligible risk that the default compiler on your system could
have bugs. We have tried our best to test and refuse to use known bad
versions in `cmake`, but we strongly recommend that you run through
the tests yourself. It only takes a few minutes, after which you can
trust your build.

The simplest way to run the checks is to build GROMACS with
`-DREGRESSIONTEST_DOWNLOAD`, and run `make check`.
GROMACS will automatically download and run the tests for you.
Alternatively, you can download and unpack the tarball yourself from
<http://gerrit.gromacs.org/download/regressiontests-@REGRESSIONTEST_VERSION@.tar.gz>,
and use the advanced `cmake` option `REGRESSIONTEST_PATH` to
specify the path to the unpacked tarball, which will then be used for
testing. If the above does not work, then please read on.

The regression tests are available from the GROMACS website and ftp
site.  Once you have downloaded them, unpack the tarball, source
`GMXRC` as described above, and run `./gmxtest.pl all`
inside the regression tests folder. You can find more options
(e.g. adding `double` when using double precision, or
`-only expanded` to run just the tests whose names match
"expanded") if you just execute the script without options.

Hopefully, you will get a report that all tests have passed. If there
are individual failed tests it could be a sign of a compiler bug, or
that a tolerance is just a tiny bit too tight. Check the output files
the script directs you too, and try a different or newer compiler if
the errors appear to be real. If you cannot get it to pass the
regression tests, you might try dropping a line to the gmx-users
mailing list, but then you should include a detailed description of
your hardware, and the output of `mdrun -version` (which contains
valuable diagnostic information in the header).

A build with `-DGMX_BUILD_MDRUN_ONLY` cannot be tested with
`make check` from the build tree, because most of the tests
require a full build to run things like `grompp`. To test such an
mdrun fully requires installing it to the same location as a normal
build of GROMACS, downloading the regression tests tarball manually
as described above, sourcing the correct `GMXRC` and running the
perl script manually. For example, from your GROMACS source
directory:

    $ mkdir build-normal
    $ cd build-normal
    $ cmake .. -DCMAKE_INSTALL_PREFIX=/your/installation/prefix/here
    $ make -j 4
    $ make install
    $ cd ..
    $ mkdir build-mdrun-only
    $ cd build-mdrun-only
    $ cmake .. -DGMX_MPI=ON -DGMX_GPU=ON -DGMX_BUILD_MDRUN_ONLY=ON -DCMAKE_INSTALL_PREFIX=/your/installation/prefix/here
    $ make -j 4
    $ make install
    $ cd /to/your/unpacked/regressiontests
    $ source /your/installation/prefix/here/bin/GMXRC
    $ ./gmxtest.pl all -np 2

If your `mdrun` program has been suffixed in a non-standard way, then
the `./gmxtest.pl -mdrun` option will let you specify that name to the
test machinery. You can use `./gmxtest.pl -double` to test the
double-precision version. You can use `./gmxtest.pl -crosscompiling`
to stop the test harness attempting to check that the programs can
be run.


## Testing GROMACS for performance ##
We are still working on a set of benchmark systems for testing
the performance of GROMACS. Until that is ready, we recommend that
you try a few different parallelization options, and experiment with
tools such as `gmx tune_pme`.

## Having difficulty? ##
You are not alone - this can be a complex task! If you encounter a
problem with installing GROMACS, then there are a number of
locations where you can find assistance. It is recommended that you
follow these steps to find the solution:

1. Read the installation instructions again, taking note that you
   have followed each and every step correctly.

2. Search the GROMACS website and users emailing list for information
   on the error. Adding
   "site:https://mailman-1.sys.kth.se/pipermail/gromacs.org_gmx-users"
   to a Google search may help filter better results.

3. Search the internet using a search engine such as Google.

4. Post to the GROMACS users emailing list gmx-users for
   assistance. Be sure to give a full description of what you have
   done and why you think it did not work. Give details about the
   system on which you are installing.  Copy and paste your command
   line and as much of the output as you think might be relevant -
   certainly from the first indication of a problem. In particular,
   please try to include at least the header from the mdrun logfile,
   and preferably the entire file.  People who might volunteer to help
   you do not have time to ask you interactive detailed follow-up
   questions, so you will get an answer faster if you provide as much
   information as you think could possibly help. High quality bug
   reports tend to receive rapid high quality answers.

# Special instructions for some platforms #

## Building on Windows ##

Building on Windows using native compilers is rather similar to
building on Unix, so please start by reading the above. Then, download
and unpack the GROMACS source archive. Make a folder in which to do
the out-of-source build of GROMACS. For example, make it within the
folder unpacked from the source archive, and call it `build-gromacs`.

For CMake, you can either use the graphical user interface provided on
Windows, or you can use a command line shell with instructions similar
to the UNIX ones above. If you open a shell from within your IDE
(e.g. Microsoft Visual Studio), it will configure the environment for
you, but you might need to tweak this in order to get either a 32-bit
or 64-bit build environment. The latter provides the fastest
executable. If you use a normal Windows command shell, then you will
need to either set up the environment to find your compilers and
libraries yourself, or run the `vcvarsall.bat` batch script provided
by MSVC (just like sourcing a bash script under Unix).

With the graphical user interface, you will be asked about what
compilers to use at the initial configuration stage, and if you use
the command line they can be set in a similar way as under UNIX. You
will probably make your life easier and faster by using the new
facility to download and install FFTW automatically.

For the build, you can either load the generated solutions file into
e.g. Visual Studio, or use the command line with `cmake --build` so
the right tools get used.

## Building on Cray ##

GROMACS builds mostly out of the box on modern Cray machines, but
* you may need to specify the use of static or dynamic libraries
  (depending on the machine) with `-DBUILD_SHARED_LIBS=off`,
* you may need to set the F77 environmental variable to `ftn` when
  compiling FFTW,
* you may need to use `-DCMAKE_SKIP_RPATH=YES`, and
* you may need to modify the CMakeLists.txt files to specify the
  `BUILD_SEARCH_END_STATIC` target property.

## Building on BlueGene ##

### BlueGene/Q ###

There is currently native acceleration on this platform for the Verlet
cut-off scheme. There are no plans to provide accelerated kernels for
the group cut-off scheme, but the default plain C kernels will work
(slowly).

Only static linking with XL compilers is supported by GROMACS. Dynamic
linking would be supported by the architecture and GROMACS, but has no
advantages other than disk space, and is generally discouraged on
BlueGene for performance reasons.

Computation on BlueGene floating-point units is always done in
double-precision. However, mixed-precision builds of GROMACS are still
normal and encouraged since they use cache more efficiently. The
BlueGene hardware automatically converts values stored in single
precision in memory to double precision in registers for computation,
converts the results back to single precision correctly, and does so
for no additional cost. As with other platforms, doing the whole
computation in double precision normally shows no improvement in
accuracy and costs twice as much time moving memory around.

You need to arrange for FFTW to be installed correctly, following the
above instructions.

`mpicc` is used for compiling and linking. This can make it awkward to
attempt to use IBM's optimized BLAS/LAPACK called ESSL (see the
section on
[linear algebra libraries](#linear-algebra-libraries)). Since mdrun is
the only part of GROMACS that should normally run on the compute
nodes, and there is nearly no need for linear algebra support for
mdrun, it is recommended to use the GROMACS built-in linear algebra
routines - it is rare for this to run slowly.

The recommended configuration is to use

    cmake .. -DCMAKE_TOOLCHAIN_FILE=Platform/BlueGeneQ-static-XL-CXX \
             -DCMAKE_PREFIX_PATH=/your/fftw/installation/prefix \
             -DGMX_MPI=ON \
             -DGMX_BUILD_MDRUN_ONLY=ON
    make
    make install

which will build a statically-linked MPI-enabled mdrun for the compute
nodes. Otherwise, GROMACS default configuration behaviour applies.

It is possible to configure and make the remaining GROMACS tools with
the compute-node toolchain, but as none of those tools are MPI-aware
and could then only run on the compute nodes, this would not normally
be useful. Instead, these should be planned to run on the login node,
and a separate GROMACS installation performed for that using the login
node's toolchain - not the above platform file, or any other
compute-node toolchain.

Note that only the MPI build is available for the compute-node
toolchains. The GROMACS thread-MPI or no-MPI builds are not useful at
all on BlueGene/Q.

### BlueGene/P ###

There is currently no SIMD support on this platform and no plans to
add it. The default plain C kernels will work.

### Fujitsu PRIMEHPC ###

This is the architecture of the K computer, which uses Fujitsu
`Sparc64VIIIfx` chips. On this platform, GROMACS @PROJECT_VERSION@ has
accelerated group kernels, no accelerated Verlet kernels, and a custom
build toolchain.

### Intel Xeon Phi ###

GROMACS @PROJECT_VERSION@ has preliminary support for Intel Xeon Phi. Only symmetric
(aka native) mode is supported. GROMACS is functional on Xeon Phi, but
it has so far not been optimized to the same level as other
architectures have. The performance depends among other factors on the
system size, and for
now the performance might not be faster than CPUs. Building for Xeon
Phi works almost as any other Unix. See the instructions above for
details. The recommended configuration is

    cmake .. -DCMAKE_TOOLCHAIN_FILE=Platform/XeonPhi
    make
    make install

# Tested platforms #

While it is our best belief that GROMACS will build and run pretty
much everywhere, it is important that we tell you where we really know
it works because we have tested it. We do test on Linux, Windows, and
Mac with a range of compilers and libraries for a range of our
configuration options. Every commit in our git source code repository
is currently tested on x86 with gcc versions ranging from 4.4 through
4.7, and versions 12 and 13 of the Intel compiler as well as Clang
version 3.1 through 3.4. For this, we use a variety of GNU/Linux
flavors and versions as well as recent version of Mac OS X.  Under
Windows we test both MSVC and the Intel compiler. For details, you can
have a look at the continuous integration server at
<http://jenkins.gromacs.org>.

We test irregularly on ARM v7, BlueGene/Q, Cray, Fujitsu PRIMEHPC, Google
Native Client and other environments, and with other compilers and
compiler versions, too.