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LHAPDF Documentation

Introduction

LHAPDF is a general purpose C++ interpolator, used for evaluating PDFs from discretised data files. Previous versions of LHAPDF were written in Fortran 77/90 and are documented at http://lhapdf.hepforge.org/lhapdf5/.

LHAPDF6 vastly reduces the memory overhead of the Fortran LHAPDF (from gigabytes to megabytes!), entirely removes restrictions on numbers of concurrent PDFs, allows access to single PDF members without needing to load whole sets, and separates a new standardised PDF data format from the code library so that new PDF sets may be created and released easier and faster. The C++ LHAPDF6 also permits arbitrary parton contents via the standard PDG ID code scheme, is computationally more efficient (particularly if only one or two flavours are required at each phase space point, as in PDF reweighting), and uses a flexible metadata system which fixes many fundamental metadata and concurrency bugs in LHAPDF5.

Compatibility routines are provided as standard for existing C++ and Fortran codes using the LHAPDF5 and PDFLIB legacy interfaces, so you can keep using your existing codes. But the new interface is much more powerful and pleasant to work with, so we think you'll want to switch once you've used it!

Installation

The latest version of LHAPDF is 6.1.4.

The source files can be downloaded from https://www.hepforge.org/downloads/lhapdf

Full installation and troubleshooting details are given at Installation instructions .

Official PDF sets

In the move to LHAPDF6 we have migrated the most recent PDF set families of each major PDF fitting group from LHAPDF5 to the new unified LHAPDF6 format. With a few exceptions for particularly significant older PDF sets, this means an effective age cutoff of ~2005 on the migrated sets.

If you need older sets you are at first encouraged either to continue with LHAPDF5, or request a data migration if you really want to use LHAPDF6. You can do the migration yourself: see the Instructions for building and running migration scripts/programs page.

The following link takes you to the PDF sets which are currently available and officially supported:

The PDF set data files can be downloaded from https://www.hepforge.org/archive/lhapdf/pdfsets/6.1/ and the latest versions are installed (as tarballs and expanded into directories) on CERN AFS at /afs/cern.ch/sw/lcg/external/lhapdfsets/current/ and CERN CVMFS at /cvmfs/sft.cern.ch/lcg/external/lhapdfsets/current/.

Please contact the LHAPDF authors at lhapd.nosp@m.f@pr.nosp@m.oject.nosp@m.s.he.nosp@m.pforg.nosp@m.e.or.nosp@m.g if you want to add new PDF sets to the standard collection. You are expected to create and validate these PDF data files yourself, and to fix and update them in the case of user-reported bugs in the public files.

Usage

As for previous versions, LHAPDF6 is a programmatic library. It can be accessed via user code written in C++, and to a more limited extent from Python and Fortran programs.

PDF sets (each of which is stored in a unique filesystem directory) should usually be installed in the $prefix/share/LHAPDF/ directory (i.e. the PDF dirs are at the same level as the global lhapdf.conf file). To make use of PDF sets installed in other places, those search paths should be listed in the LHAPDF_DATA_PATH environment variable. [LHAPDF_DATA_PATH is an extension of the old LHAPATH variable which supports multiple search paths separated by colon (:) characters, cf. standard system paths like PATH, LD_LIBRARY_PATH, etc.]

Here are some reference documents on the library design and the system of PDF/set/config metadata flags:

Many code usage examples are collected here:

Trick to remove unwanted PDF members

A typical LHAPDF6 PDF set with error members is between 20 and 100 MB in size. If you only intend to use a subset of these (e.g. in MC production where you will only use the central member), you can simply remove the unneeded .dat files from the PDF set directory. It will work fine, as long as you don't try to load one of the missing members of course! In grid job submissions this can reduce the tarball of data that you send with your job by a large amount, since PDF sets often dominate such job tarballs.

Trick to use zipped data files

For special applications where PDF initialization speed or disk space are very critical, you may wish to use zipped PDF data files. Since the data files are plain text, they compress well, by a factor of 4 or so, but for most applications this is largely irrelevant since you will usually not need to install more than about 100 MB of data files to use the main modern PDF sets, and set initialisation time is less than one second... and that happens only once per run.

Actually reading zipped data files would make LHAPDF's code and (more importantly) build process much more awkward, so we haven't done that. However, if you really need to squeeze the maximum out of limited space and time then there is a trick you can play – at least on Linux systems – using LD_PRELOAD. This environment variable specifies libraries to be loaded before starting a process, and is a mechanism for replacing system library functions with modified versions.

In this case, we can override the open("foo") function with a version which will attempt to transparently read foo.gz, uncompressed into the /tmp directory, by setting LD_PRELOAD=/lib/uncompress.so in your job. See the zlibc documentation for more details. LHAPDF6 itself needs no modification, but you will need to cd into your PDF sets and run for i in *.dat; do gzip $i; done in each one that you want to compress.

This is rather a hack and may not work on Macs... but it can be useful in special circumstances. Please let us know your experiences if you use it!

Authors

LHAPDF6 AUTHORS
===============
Lead developer:
Andy Buckley
Code contributors:
James Ferrando
David Hall
Steve Lloyd
Karl Nordstrom
Martin Ruefenacht
Graeme Watt
Testing, advice, etc.:
Richard Ball
Luigi del Debbio
Nathan Hartland
Ben Page
Juan Rojo
Marek Schoenherr
Mike Whalley
CITATION
========
Please cite the contribution
"New access to PDF data via LHAPDF6", by A. Buckley
in "Les Houches 2013: Physics at TeV Colliders: Standard Model Working Group Report"
http://arxiv.org/abs/1405.1067
http://inspirehep.net/search?p=find+eprint+1405.1067
A full write-up of LHAPDF6 will appear as soon as possible and
will replace this current recommendation.

Support and bug reporting

If you need assistance with LHAPDF, please contact the authors at lhapd.nosp@m.f@pr.nosp@m.oject.nosp@m.s.he.nosp@m.pforg.nosp@m.e.or.nosp@m.g . Please also send feature suggestions to this address: please don't contact individual developers if you can avoid it. We also accept compliments ;-)

You are very strongly advised to make sure that you are running the latest version of the LHAPDF library, since issues are often fixed in later releases. Please supply some information about which version you are using, what type of system and compiler you are using, a copy of the LHAPDF config.log file, a reasonably full copy of the errors you are getting, and the output of downloading and running this script: https://users.hepforge.org/~buckley/sysdebug.sh .

Please do not send us screenshots of your terminal! To capture the output of a command, use a shell redirection like this: make &> makelog.txt or bash sysdebug.sh &> sysdebug.txt (you may prefer to use a redirection like |& tee foolog.txt but probably you don't need our advice if you have such preferences!)

Lastly, please try to read the configure/compiler/runtime error message: once decoded it may tell you something simple e.g. you have not got the required compiler, you are compiling against a 32 bit library on a 64 bit system, or you have not installed a PDF data file. If you can't parse the error messages then try Googling for generic-looking bits of the message before asking for direct assistance: there are often good explanations and solutions online.

For developers

To check out LHAPDF6 from Mercurial, use this command for read-only: hg clone https://lhapdf.hepforge.org/hg/lhapdf, and this for read-write: hg clone ssh://USER@login.hepforge.org//hepforge/hg/lhapdf/public/lhapdf (requires a HepForge login account; replace USER with your actual HF user name.)

See the following pages:

Todo:
Add some developer build guides, including the autotools, Cython, etc. gotchas.