  The dependence of the magnetocrystalline anisotropy energy (MAE) of MCo5 (M =
Y, La) on the Coulomb correlations and strength of spin orbit (SO) interaction
within the GGA + U scheme is investigated. A range of parameters suitable for
the satisfactory description of key magnetic properties is determined. The
origin of MAE in these materials is mostly related to the large orbital moment
anisotropy of Co atoms on the 2c crystallographic site. Dependence of
relativistic effects on Coulomb correlations, applicability of the second order
perturbation theory for the description of MAE and effective screening of the
SO interaction in these systems are discussed using a generalized virial
theorem.
