  We introduce HP, an implementation of density-functional perturbation theory,
designed to compute Hubbard parameters (on-site $U$ and inter-site $V$) in the
framework of DFT+$U$ and DFT+$U$+$V$. The code does not require the use of
computationally expensive supercells of the traditional linear-response
approach; instead, unit cells are used with monochromatic perturbations that
significantly reduce the computational cost of determining Hubbard parameters.
HP is an open-source software distributed under the terms of the GPL as a
component of Quantum ESPRESSO. As with other components, HP is optimized to run
on a variety of different platforms, from laptops to massively parallel
architectures, using native mathematical libraries (LAPACK and FFTW) and a
hierarchy of custom parallelization layers built on top of MPI. The
effectiveness of the code is showcased by computing Hubbard parameters
self-consistently for the phospho-olivine Li$_x$Mn$_{1/2}$Fe$_{1/2}$PO$_4$
($x=0, 1/2, 1$) and by highlighting the accuracy of predictions of the geometry
and Li intercalation voltages.
