  In this work we suggest a theoretical approach, that allows to study the
effects of magnetocrystalline anisotropy (MCA) in metallic systems using the
Green`s functions formalism. We demonstrate that employment of the reciprocal
space resolution instead of its reduction in the inter-site variant essentially
improves the numerical stability of MCA energy by means of Monkhorst-Pack grid
density and spatial convergence. The latter problem is able to be completely
removed due to rigorous analytical replacement of pairwise atomic summation by
simple composition of sublattices contributions, calculated as a whole. The
approach is validated on the effective model of single atom, which nevertheless
inherits the qualitative MCA picture of Co monolayer and Au/Co/Au sandwiched
material. The numerical convergence is confirmed using the model of atomic
chain in the strong metallic regime. For cobalt monoxide, described by ab
initio calculations using GGA+U, the MCA energy angular profile reveals the
prevailing role of ferromagnetically aligned Co sublattices in forming of the
easy axis.
