Theoretical design of new organic magnetic materials usually requires that calculations of electronic structure and optimal geometry are conducted for quite large poly radicals. Due to spin contamination, conventional unrestricted density functional theory (UDFT) is a rather crude tool for such studies. We have developed a simple modification of the UDFT, which yields very reliable results for the ground state spin of large poly radicals. This method was applied to calculation of electronic structure and excitation spectra for various fragments of a ferrocarbon-ferromagnetic phase of pure carbon- which has been synthesized in many laboratories during the last decade.