Spin-orbit concept of open-shell systems

The paper presents a first attempt to look at the peculiarities of open-shell systems from the viewpoint of spin-orbit coupling. Contrary to common opinion of a negligible role of the issue for light element molecules, presented in the paper evidences its governing role in such systems peculiarities on example of sp2 nanocarbons. A particular attention is given to the justification of the validity of UHF approach to exhibit the SOC peculiarities of open-shell systems.