========================================================================================
=  Paper describing the HP code:                                                       =
=  - I. Timrov, N. Marzari and M. Cococcioni,                                          =
=    "HP - A code for the calculation of Hubbard parameters using density-functional   =
=     perturbation theory", Comput. Phys. Commun. 279, 108455 (2022); arxiv:2203.15684 =  
========================================================================================
=  The calculation of Hubbard parameters using the HP code is based on DFPT:           =
=  - I. Timrov, N. Marzari and M. Cococcioni,                                          =
=    "Hubbard parameters from density-functional perturbation theory",                 =
=    Phys. Rev. B 98, 085127 (2018); arXiv:1805.01805                                  =
=  - I. Timrov, N. Marzari and M. Cococcioni,                                          =
=    "Self-consistent Hubbard parameters from density-functional perturbation theory   =
=     in the ultrasoft and projector-augmented wave formulations",                     =
=    Phys. Rev. B 103, 045141 (2021); arXiv:2011.03271                                 =
========================================================================================

Some examples of the application of the HP code:
- C. Ricca, I. Timrov, M. Cococcioni, N. Marzari, and U. Aschauer,
  "Self-consistent site-dependent DFT+U study of stoichiometric and defective SrMnO3",
  Phys. Rev. B 99, 094102 (2019); arXiv:1811.10858
- C. Ricca, I. Timrov, M. Cococcioni, N. Marzari, and U. Aschauer,
  "Self-consistent DFT+U+V study of oxygen vacancies in SrTiO3",
  Phys. Rev. Research 2, 023313 (2020); arXiv:2004.04142
- I. Timrov, P. Agrawal, X. Zhang, S. Erat, R. Liu, A. Braun, M. Cococcioni,
  M. Calandra, N. Marzari, and D. Passerone,
  "Electronic structure of pristine and Ni-substituted LaFeO3 from near edge x-ray 
   absorption fine structure experiments and first-principles simulations",
  Phys. Rev. Research 2, 033265 (2020); arXiv:2004.04142
- I. Timrov, F. Aquilante, L. Binci, M. Cococcioni, and N. Marzari,
  "Pulay forces in density-functional theory with extended Hubbard functionals: 
   From nonorthogonalized to orthogonalized manifolds",
  Phys. Rev. B 102, 235159 (2020); arXiv:2010.13485 
- N.E. Kirchner-Hall et al., 
  "Extensive Benchmarking of DFT+U Calculations for Predicting Band Gaps",
  Appl. Sci. 11, 2395 (2021)
- Y. Xiong et al., 
  "Optimizing accuracy and eﬃcacy in data-driven materials discovery for the solar 
  production of hydrogen",
  Energy Environ. Sci. 14, 2335 (2021)
- J.-J. Zhou et al.,
  "Ab Initio Electron-Phonon Interactions in Correlated Electron Systems",
  Phys. Rev. Lett. 127, 126404 (2021)
- R. Mahajan et al.,
  "Importance of intersite Hubbard interactions in beta-MnO2: A first-principles DFT+U+V study",
  Phys. Rev. Materials 5, 104402 (2021)

Note: The DFPT approach (as the linear-response cDFT approach) has a limitation:
it is applicable only to open-shell systems. For more details see
K. Yu and E.A. Carter, J. Chem. Phys. 140, 121105 (2014).

