  The correlation of odd electrons in graphene turns out to be significant so
that the species should be attributed to correlated ones. This finding
profoundly influences the computational strategy addressing it to
multireference computational schemes. Owing to serious problems related to the
schemes realization, a compromise can be suggested by using single-determinant
approaches based on either Hartree-Fock or Density-Functional theory in the
form of unrestricted open-shell presentation. Both computational schemes enable
to fix the electron correlation, while only the Hartree-Fock theory suggests a
set of quantities to be calculated that can quantitatively characterize the
electron correlation and be used for a quantitative description of such
graphene properties as magnetism, chemical reactivity, and mechanical response.
The paper presents concepts and algorithms of the unrestricted Hartree-Fock
theory applied for the consideration of magnetic properties of nanographenes,
their chemical modification by the example of stepwise hydrogenation, as well
as a possible governing the electron correlation by the carbon skeleton
deformation.
