The LDA and semilocal GGA functionals often fail to describe systems with localized (strongly correlated) or electrons (this manifests itself primarily in the form of unrealistic one-electron energies or too small magnetic moments). In some cases this can be remedied by introducing on the or atom a strong intra-atomic interaction in a simplified (screened) Hartree-Fock like manner (), as an on-site replacement of the LDA/GGA functional:
where is the double-counting term and is the on-site occupancy matrix of the or electrons. This approach is known as the DFT+U method (traditionally called LSDA+U).
The first VASP DFT+U calculations, including some additional technical details on the VASP implementation, can be found in Ref.  (the original implementation was done by Olivier Bengone  and Georg Kresse).
For more details, read the article about the formalism of the DFT+U method.
DFT+U can be switched on with the LDAU tag, while the LDAUTYPE tag determines the DFT+U flavor that is used. LDAUL specifies the -quantum number for which the on-site interaction is added, and the effective on-site Coulomb and exchange interactions are set (in eV) with the LDAUU and LDAUJ tags, respectively. Note that it is recommended to increase LMAXMIX to 4 for d-electrons or 6 for f-elements.