\section{Installation}
\label{sec:Installation}

The \MCFM{} package may be downloaded from the \MCFM{} homepage at \url{https://mcfm.fnal.gov}.
After extracting, in the simplest case, the source can be compiled by running \texttt{cmake ..} in 
the Bin directory:
\begin{lstlisting}
tar -xzvf MCFM-X.Y.tar.gz
cd mcfm/Bin
cmake -DCMAKE_Fortran_COMPILER=gfortran -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++  ..
\end{lstlisting}
We require at least \texttt{gcc}/\texttt{g++}/\texttt{gfortran} of version 7 or 
greater, and a \texttt{cmake} 
version greater than 3. By default all necessary dependencies, including LHAPDF, are compiled with
the default compiler that \texttt{cmake} detects. To adjust the compiler, or disable the use of the 
internal \LHAPDF{}, or enable {\abbrev MPI}, we refer to the \texttt{INSTALL} file.

Additional complications may arise especially on OS X systems, where by default 
\texttt{gcc}/\texttt{g++}
is linked to the clang compiler. Please make sure to follow the \texttt{INSTALL} instructions to
change the compiler names to the GNU versions. Note that \LHAPDF{} 6.3.0 has a multithreading
bug and we therefore we do not recommend to use it. \MCFM{} has been tested with \LHAPDF{} 6.2.X. 
Version 5 of \LHAPDF{} is not supported.

Please ensure that your compiler is working and can produce executable program
files. For example when your compiler has been installed into a non-standard
location you probably need to append the compiler library path to {\tt
LD\_LIBRARY\_PATH } ({\tt DYLD\_FALLBACK\_LIBRARY\_PATH} on OS X).  This can be
achieved, for example, as follows:
\begin{verbatim}
export LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:/home/user/local/lib/gcc7
\end{verbatim}

The directory structure of \MCFM{} is as follows:
\begin{itemize}
\item {\tt Doc}. The source for this document.
\item {\tt Bin}. The directory containing the executable {\tt mcfm},
and various essential files -- notably the options file {\tt input.ini}.
\item {\tt Bin/Pdfdata}. The directory containing the internal \PDF{} data-files.
\item {\tt Bin/PDFs}. Directory for \LHAPDF{} grid files used by bundled \LHAPDF{}.
\item {\tt src}. The Fortran source files in various subdirectories.
\item {\tt lib/TensorReduction} General tensor reduction code based on the work of Passarino and 
Veltman \cite{Passarino:1978jh} and Oldenborgh and Vermaseren \cite{vanOldenborgh:1989wn}.
\item {\tt lib/qcdloop-2.0.5}. The source files to the library 
QCDLoop~\cite{Carrazza:2016gav,Ellis:2007qk}.
\item {\tt lib/oneloop}. The source files to the library OneLOop~\cite{vanHameren:2010cp}.
\item {\tt lib/qd-2.3.22}. Library to support double-double and quad-double precision data types \cite{libqd}.
\item {\tt lib/AMOS}. Library for AMOS, `` A Portable Package for Bessel Functions of a Complex Argument
 and Nonnegative Order'', taken from {\tt http://www.netlib.org/amos/}.
\item {\tt lib/SpecialFns}. Library containing the implementation of special
functions from a variety of sources.
\item {\tt lib/VVamp}. Library containing the implementation of two-loop helicity
amplitudes for $q\bar q \to V_1 V_2$, from the results of
Ref.~\cite{Gehrmann:2015ora}.
\item The {\tt lib/handyG} library for the evaluation of generalized
polylogarithms~\cite{Naterop:2019xaf}.
\end{itemize}
 
\subsection{OpenMP and MPI}
\MCFM{} uses \OMP{} (Open Multi-Processing) to implement multi-threading and automatically adjusts to the 
number of available \CPU{} threads. The multi-threading is implemented with respect to the integration routine 
Vegas, which distributes the event evaluations over the threads and combines all events at the end of 
every iteration.

Two environment variables are useful. On some systems, depending on the \OMP{} implementation,
the program will crash when calculating some of the more complicated processes,
for example $W+2$~jet production at \NLO{}.
Then, adjusting {\tt OMP\_STACKSIZE} may be needed for the program to run correctly.
Setting thisvariable to {\tt 16000}, for instance in the Bash shell by using the
command {\tt export OMP\_STACKSIZE=16000}, has been found to be sufficient
for all processes.  The second useful variable {\tt OMP\_NUM\_THREADS}
may be used to directly control the number of threads used during
\OMP{} execution (the default is the maximum number of threads available
on the system).

It is also possible to run \MCFM{} using \MPI{} (Message Passing Interface).
To run in this mode, change the flag {\tt USEMPI} in the makefile to {\tt YES} and specify the MPI compiler 
wrappers and compilers in the makefile or set the environment variables \texttt{FC} and \texttt{CXX}.
By default, the OpenMPI compiler wrappers mpifort and mpic++ are used,
to use gfortran and g++. When {\tt USEINTEL} is set, mpiifort and mpicc are used.