```eval_rst
.. _lhapdf:
```
# PDF set storage and interpolation
[LHAPDF](https://lhapdf.hepforge.org/) is a C++ library that evaluates PDFs by interpolating the
discretised PDF 'grids' that PDF collaborations produce. It also gives its users access to proton
and nuclear PDF sets from a variety of PDF collaborations, including NNPDF, MMHT and CTEQ. A list
of all currently available PDF sets can be found on their
[website](https://lhapdf.hepforge.org/pdfsets.html). Particle physics programmes that typically make
use of PDFs, such as Monte Carlo event generators, will usually be interfaced with LHAPDF, to allow
a user to easily specify the PDF set that they wish to use in their calculations. You can read more
about LHAPDF by reading the [paper](https://arxiv.org/abs/1412.7420) that marked their latest
release.
## PDF evolution
[APFEL](https://apfel.hepforge.org/) ('A PDF Evolution Library') is the PDF evolution code currently
used by the NNPDF Collaboration. In addition to its PDF evolution capabilities, it also produces
predictions of deep-inelastic scattering structure functions. In recent years it has been developed
alongside NNPDF, and so it therefore contains the features and settings required in an NNPDF fit.
That is, it includes quark masses in the MSbar scheme, the various FONLL heavy quark schemes, scale
variations up to NLO, etc. Note that at the time of writing, a more streamlined code is being
written to replace APFEL, which is currently dubbed EKO ('Evolution Kernel Operator'). To find more
general information about PDF evolution and the DGLAP equations, you can go to the [Theory
section](dglap.md).
## PDF compression
PDF compression seeks to maintain the statistical accuracy of a large sample of replicas
produced by a fit when using a PDF set with a smaller number of replicas (and thus fewer
convolutions required to compute cross sections with PDF uncertainties). For example the
main published PDFs are typically based on a 1000 replica fit, which can then be compressed to
around a 100 replicas PDF set while maintaining good accuracy of most relevant statistical estimators.
This is done with the [pyCompressor](https://n3pdf.github.io/pycompressor/) library,
a python compression code that extracts, from an initial PDF set of replicas,
the subset that most truthfully reproduces the underlying probability distribution of the prior.
[pyCompressor](https://n3pdf.github.io/pycompressor/) is an updated python version of
[compressor](https://github.com/scarrazza/compressor), which was used in previous releases.
### Other codes
[Hoppet](https://hoppet.hepforge.org/) ('Higher Order Perturbative Parton Evolution Toolkit') is an
alternative PDF evolution code which is capable of evolving unpolarised PDFs to NNLO and linearly
polarised PDFs to NLO. The unpolarised evolution includes heavy-quark thresholds in the MSbar
scheme.